Just a little disclaimer, I won't cover any of the Special Characters. That's something I never do and I don't plan on ever doing it.
Mandatory Choices
HQ
Everyone one of these heroic heroes can purchase items from the Remnants of Glory (think Special Issue wargear from our book), except for Warlocks. Also any of these units (except Warlocks) can join your Tau units.Farseers: Probably your primary choice for this slot. These guys are very powerful with their wide variety of powers and magnificent support powers that will dramatically boost the power of any Tau gun line. Don't forget about the jetbike you can throw this heroic Seer on.
So many of the psychic powers that this Seer holds will be very useful including the Primaris Power for two different "schools" of powers has Guide which provides the target unit with re-rolls to hit. Think of the devastation that a twin-linked, Fire Storm squad of Fire Warriors within rapid fire range.
Not really potent in close combat or at range combat, but this ally can unleash horrific powers upon your foe using the bizarre gifts provided to it as well as guiding your very own Tau to their maximum potential.
Autarch: A powerful master of weapons and has a wide variety of tools at his disposal ranging from helmets, modes of transportation and weapons. Not the best support ally available, but a very powerful ally when it comes down to range or close combat. This is an extremely versatile option like the Suits of the empire.
Kit the guy up with a jetbike, Banshee Mask, Fire Sabre (similar to a special issue item for Tau) and you've got a very quick and deadly anti-marine close combat unit running around. Not only can this be a powerful close combat unit, you can give him some very deadly range weapons along with the close combat weapons and a jetbike. Throw in a Reaper Launcher or even an Uldanorethi Long Rifle.
Throw in his ability to manipulate reserves and now he's also a tactical choice as well. Use this to bring in your outflankers or deepstrikers whenever you want.
You can turn the guy into a jetbike unit, jump infantry or even jetpack infanty. Don't forget, he can even join your suit squads too! In fact, kit him with the Warp Jump Generator and whatever guns you want to add some firepower. Give him the Swooping Hawk wings, a close combat weapon and then pair him up with your Vespids for some Hit and Run action.
Spiritseers: Maybe not the best choice for a psyker due to his more limited amount of powers, one less wound, and one less level to his mastery, but he is a great part to your ally detachment if you plan on using any Wraith units (can mark units for all Wraigh units to re-roll all to hits of 1) especially any Wraithblades, or Wraithguards (makes them troops).
They also have access to the Runes of Battle powers which the Farseer doesn't and these are the fabled powers that have two uses. One of these powers can give a unit shrouded or remove stealth and shrouded from an opponent. Another of these is destructor (which is essecntially a psychic heavy flamer) or an ally regains a wound.
Other than that, this guy really isn't anything too fancy. If you just want some psychic powers than go with the Farseer instead.
Warlock Council: They all have access to the runes of battle which make them very powerful allies indeed, but they can only join Guardian squads (foot guardians, jetbikes or Support Batteries) or they can sit around in a squad of up to 10 of these deadly fellows. Load them all up on jetbikes, slap on a Farseer or an Autarch and you have one of the most powerful Deathstar units to date. Just watch out for Perils of the Warp seeing how they don't have any defenses what so ever against it.
This guy will literally melt any foe in close combat as well as inspire any friendly Eldar units nearby. Also, this fellow can purchase a few upgrades to make him a truly powerful close range and close combat unit.
Troops
Dire Avengers: One of the better troop choices ever (besides Marines), these fellows can have some truly potent close combat options along with invulnerable saves for them selves or the whole unit. They can even take power weapons (emphasis on power weapons; whatever you model, is whatever you get).They also have one of the longest ranges compared to most of the other troop choices as well as move, shoot, and then run or move, run and then shoot, to truly close the range gap that many of the Eldar struggle with. Their guns are two shots, 18" range and have rather bizarre type of rending that's even more potent against infantry than before.
Guardian Defenders: A powerful unit that can be accompanied by a Warlock and they can utilize one of the many heavy weapons available to the Eldar. Perfect for sitting on top of an objective while the rest of your Tau units are advancing and firing upon the targets. They can also have up to 20 foot soldiers, one Warlock and two heavy weapons for extra umpth and sturdiness.
One of their downsides is their extremely short range weapons, but they do have a more potent version of rending against infantry.
Storm Guardians:.......Not one of your best decisions. I must say that there are plenty of better ideas than this one. Not saying it's a bad one just not really a good one to complement your Tau army.
At least they have assault grenades (Banshees!), carry two power swords max, or two fusion guns/flamers max and even 20 troopers along with a Warlock. A good choice for counter assaults or something to tie up enemy assault units with.
Windrider Jetbike: The most mobile troop choice in the whole game and maybe one of the better ones in this codex. The squad can hold up to ten men and even wield three shuriken cannons (deadlier and longer ranged versions of the weapons held by Avengers or Guardians). Also include the Warlock in (hopefully with the power that provides Shrouded for a 3+ jink save) and this is one of the best units you can wield in your ally attachment.
Don't forgot that you can also give a Farseer a jetbike as well as an Autarch to provide this squad with some really power.
Rangers: Masters of long range anti-personal warfare as well as being one of the best "castleing" (sit on an objective and hold it) units in the game. Nothing like ten Eldar sniper rounds to your face when you decide to charge an objective.
Wave Serpent: Obviously not a troop choice, but none the less essential for victory. Literally this thing carries weapons everywhere and can even out shoot a Hammerhead gunship.
Equip it with Scatter Lasers (twin-links the rest of the weapons on the vehicle), shuriken cannon and then fire off the shield. Nothing can stand unscathed from a barrage like that.
Fun Stuff
Fast Attack
Swooping Hawks: The most powerful anti-infantry choice available to the Eldar from their standard weapons to their extremely potent grenade pack. They can re-deploy over and over again as well as never scatter when deep striking. They're jump infantry rather than jetpack infantry, but they are a perfect match for Vespids.
Also, put them in the same army as an Autarch and you have even more control of when you deep strike them.
Warp Spiders: The perfect comrade to pair up with your Suits since they are jetpack infantry. Maybe not the best AP wise, but they have an extremely high strength and mobility. They can also do the same dirty "run, shoot" stuff as the rest of the Eldar to provide them with up to 18" of movement in one turn.
Shining Spears: A deadly Jetbike unit that excels at hunting tanks, close range fire power, and charging into combat. No squad of marines will be safe after this 10 men squad charges through them. Throw in an Autarch and quite literally you can kill tanks or MEQs more effectively than any other bike unit in the game.
Crimson Hunter: The glass cannon of anti-air choices. Not really fond of flyers myself, but this guy can outgun any other flyer in the game, but it will die if anything even blows on it.
Vyper Squadron: You play Tau, so you must like heavy weapons. Your allying with Eldar, so you must like mobility. These guys are the living embodiment of just this. Sure they're fragile (not as fragile as they used to be though due to all the new upgrades), but they can carry some very deadly weapons, especially since they're only Fast Attacks.
Hemlock Wraithfighter: Perfect unit if you like to play dirty. This guy can constantly strike fear into your opponents' hearts until they run off the battlefield allowing you to win a war of attrition. Still highly fragile like the Crimson Hunter.
Elites
Howling Banshees: Truly a worthless assault unit, but a perfect counter assault choice. Works great when paired with your gun line to provide that extra power that they need in close combat after they've been charged.Striking Scorpians: Still not the most powerful assault unit ever, but still very effective with infiltrate, assault grenades and a power fist that doesn't reduce the Exarch's initiative. Also they are one of the few Eldar units to have 3+ armor. A great unit when used in conjuction with Kroot and Stealthsuits.
Harlequins: Not the best choice especially now that their Veil of Tears power is actually a power rather than an automatic thing, but still useful on the charge or counter charge. I would still recommend not wasting your Elites choice on these guys though.
Wraithguard: (can become troops when you use a Spiritseer) A truly deadly squad that can also act as a bullet magnet. They might be expensive, but they sure can kill more termies than a plasma storm squad of Suits or a Riptide nova-charged. The only downside is their extremely short range of 12" or a template. If you also take a Spiritseer, they can be one of the most powerful troop choices in the game as well as the best overwatch unit if they're equipped with their flamer variant.
Wraithblades: (can become troops when you use a Spiritseer) Another bullet magnet unit that can be somewhat potent in combat with their dual swords (anti-marine) or shields and axes (anti-termy and higher durability). Great unit to divert shots from your quintessential gun line or for holding objectives when paired with the Spiritseer.
Heavy Support
Dark Reapers: A great anti-MEQ choice and one of the few reliable anti-air choices available to Eldar. Not really all that useful compared to your Tau units and in fact they're kind of redundant.Support Battery: Some rather strange weapons here, but not the best ones ever. They have some rather short ranges compared to your Tau weapons and you should probably spend your points elsewhere. They are rather cheap though and you can field a Warlock with them too to make them a little more reliable.
Fire Prism: An extremely reliable and versatile tank option that one could argue that rivals the Hammerhead. It has three firing options; one for anti-infantry, another for anti-MEQ, and another for blowing up tanks (it's a S 9 lance weapon). Perfect partner for Hammerheads, but not a replacement.
Night Spinner: Terrific for taking out light Infantry, but not really that great of an idea considered the fact that a squad of Fire Warriors would be more potent.
Falcon: A nice tactical transport (carries 6 men) that can carry a nice arsenal of deadly weapons. Equip it with a Scatter Laser and a Shuriken Catapult to provide your army with some reliable, versatile firepower.
Probably not the best choice compared to all of these deadly Heavy Support Choices (or your Tau ones), but still a handy one if you're running Aspect Warriors in small squads and don't want to purchase a transport or a heavy tank; just mix the two.
War Walker Squadron: A mobile means of brining heavy weapons to the front where they will be needed most. Three walkers make up a single squad and each walker can carry two weapons. This makes for some extremely barrages, but they are still rather weak armor wise. They are the other land unit to have access to flakk missiles besides Reapers though, but not really needed compared to your Tau counterparts unless you just really like the models or Walkers.
Wraithlord: Keep him around your Riptide for company as well as equipping the fella with two heavy weapons, two small (useless) weapons and a giant sword. Have him run counter assault for your Riptide or even charge him towards your opponent. This should relieve some of the firepower away from the Riptide to ensure these behemoths last a little longer.
Wraighknight: A huge expenditure of points that's kind of disappointing. The model is terrific, but pretty fragile. He does carry an array of lethal weapons than can aid your Riptide in terminating those pesky Terminators. Should aid your Riptide by drawing fire away from him as well as some monster hunting assault units, but really don't spend the points or the cash on this.