Preface:
Daemons is a very, very linear codex in terms of actual options. Most codexes will allow the player's HQ to take exactly the weapons and wargear they want him to have. Not so with the Chaos Demons Codex - While you get access to good weapons in the rewards '0' options, you can't give a model a specific weapon, the way you want to. As an example, you can't pay 10 points and give your herald of khorne an Axe of Khorne. You have to buy a lesser reward, which takes up rewards points. This is an interesting technique, as it is unique to the Daemons codex and limits army-list level character variations.
Keeping this in mind, I can no longer truly offer loadout suggestions for your army. Instead, I can only suggest which rewards best suit this particular character. My "Standard Loadouts" are a suggestion of how I will usually run that unit. Also note, I won't be reviewing Daemon Princes in this section, as they're usually taken in the Heavy Support Slot.
Finally, if you feel I've undervalued something, or if I've forgotten to include something, feel free to tell me. If I agree with the statement you present me, I will change the entry accordingly.
Note: Metacharacteristic value is a completely useless stat. To find the Metacharacteristic value of a unit, add up their statline (that is, WS/BS/S/T/W/I/A/LD), then divide by 80. This is the percentage of a 'perfect statline' they meet.
9. Tzeentch 
Lord of Change:
Strengths
T6 with 5 Wounds, 5++ rerolling 1's.
Reliable (albeit Psychic) shooting
Weaknesses
Doesn't compare to the control that Kairos Fateweaver gives you.
Things to Watch:
Aim for precognition and perfect timing. If you get both, or if you just get one, go with FFoT. You'll have a very accurate 2D6 Heavy Bolter on a unit that re-rolls basically everything
Failing in that, this guy makes a very safe grimoire. Try to get the 4+ invulnerable save and your unit will get a 2+. This is the backbone of the screamerstar, as I'm sure you already knew.
Standard Loadout:
Maxed out. Always ML3, usually either 2 Greaters 1 Lesser, or Greater/lesser reward. Run him with a Staff of Change to get 6-7 S8AP2 attacks at I6 in close combat, and make your enemies explode when you kill them. Usually 305 points.
Price 8/10 For the 1/6 of your army price tag, you get a lot to play with.
Survivability 7/10 5 wounds at T6 with anywhere from a 2++ to a 5++ that also re-rolls 1's? very scary.
Damage Output 8/10 Mostly dedicated anti-FMC shooting, but a decent combatant.
Rules 6/10 Tzeentch daemons have lots of benefits, but the Psychic powers have really noticable drawbacks. Usually, the LOC is good enough that he'll annihilate your target outright rather than giving them FNP6+.
Army Bonuses 8/10 Want to make your grimoire tank? This is your dude to make it happen.
Overall 7.4/10
Metacharacteristic Value: .61
Kairos Fateweaver:
Strengths
Decent Shooting, T5 with 5 wounds and a 4++, re-rolling 1's.
One Re-roll per player turn*
Lord of Unreality Warlord Trait secured.
Weaknesses
Warlord that adds a huge amount of boons to your army. Always going to be number-one target on your enemy's shit list.
The King of Choices has not choice when it comes to his psychic powers.
He can't make use of all 8 of his psychic powers, Baaw!
Things To Watch
Kairos has access to two very interesting powers. the first is Forewarning in the Divination Discipline. We all know why this is good. the other useful one is Fire Shield - a 4+ cover save is as good as a 4++ against anyone but tau or eldar.
Standard Loadout
He's perfect as he is.
* There's Contention about Kairos Fateweaver's re-roll. This could possibly be the most broken thing in the codex because, RAW, you can re-roll any dice during your turn and your opponents, including those rolled by your opponent. Furthermore, Kairos doesn't even need to be alive for it to take effect. Of course, neither of these sound correct RAI.
Price 10/10 I might go as far as to say he's underpriced.
Survivability 6/10 So Important to some army's strategy, that his T5 becomes a real problem for him. Still, he is a T5 FMC with 5 wounds and a 4++.
Damage Output 7/10 He has it, but that's not his job. Can be useful as a flyer grounder if you need it, but avoid it.
Rules 10/10 Two Heads of Fate can either be your best friend or your worst enemy, but it's his other rules that make him great.
Army Bonuses 10/10 Daemons are all about going with the flow and taking what comes your way. Kairos Fateweaver is all about taking all that control you surrendered back.
Overall 8.6/10 Definitely the most powerful character in the codex.
Metacharacteristic Value: .43
8. Khorne
Bloodthirster
Strengths
Combat beast
Annihilate almost anything in close combat
Can get massive amounts of attacks
One of the only reliable ways of killing AV14 in the Daemons Codex
Flyer Killer Extraordinaire (Mediocre against AV12 or better)
Weaknesses
For all his strengths, he falters in the face of the Lord Of Change, that can do almost everything he can do better.
Things To Watch
Throw him at a squad with rage/rampage and a greater etherblade and watch him slaughter.
Standard Loadout
Contention over Greater/exalted for souleater/+1W/IWND combo and 2 Greater rewards for better survivability. Sucks if you get a 6 on greater though.
Price 6/10 He'd be nicely priced if the LOC wasn't only 5 points more, and fateweaver the same price.
Survivability 7/10 3+ save can be the difference between life and death, and since most FMC get AP4 or worse thrown at them to ground them, he's pretty safe with a 3+.
Damage Output 10/10 The Bloodthirster will maul anything you want him to maul. Just watch him.
Rules 4/10 He has nothing to make him better than a LOC, except maybe guaranteed rampage. Too bad he usually wants to stay in the sky.
Army Bonuses 3/10 What's an army bonus?
Overall 6/10 He struggles to compare to the LOC, but he'd still be useful in a khorne monotheme.
Metacharacteristic Value: .76
Skarbrand
Strengths
Force Multiplier for your side.
7 attacks base.
Reliable Anti-tank
Weaknesses
Force Multiplier for your enemy
No Wings, No Whip.
Needs to be Warlord to get instant death.
No Rewards.
Things To Watch
Deepstrike him using an icon and watch your units and your enemies units roll and re-roll dice until everyone's dead!
Standard Loadout
On the Shelf. No Wings? No Way.
Price 3/10 The Price Drop wasn't sufficient to make him worthwhile.
Survivability 6/10 Helps your enemy so much he won't be shooting at you ever.
Damage Output 6/10 Arguably a more flexible close combatant than your average thirster.
Rules 5/10 If we pretend not having wings is not one of his rules, we could say he has nice rules.
Army Bonuses 5/10 They're great! they just help your enemy a little too much.
Overall 5/10 A reasonable full step below the already underwhelming bloodthirster.
Metacharacteristic Value: .78
7. Nurgle 
Great Unclean One
Strengths
Your enemy is never getting warlord, if he is warlord.
Instant death if he ever closes in for combat.
5 attacks at 2+/2+ means he's usually winning combat, along with his T7.
Biomancy is a bitch.
Weaknesses
Very slow. Laughably so.
Things To Watch
Watch him get Iron arm and S9T10. Nothing will remove him while he slowly plods over to your enemy.
Standard Loadout
ML3 with Exalted Reward and Greater to make a reliable grimoire, ML3 with greater/greater/lesser to ensure your enemy never gets warlord.
Price 7/10 He's reasonable, he's just to slow to be useable.
Survivability 10/10 Wanna talk tanks?
Damage Output 6/10 Good When he gets there, AP3/2 shooting attacks can be fun.
Rules 5/10 Slow and Purposeful hurts a lot, but biomancy makes up for it. Also, unlocks Nurgle DPs.
Army Bonuses 6/10 Not having to worry about warlord is pretty nice. Good in a flying circus, if you want reserves.
Overall 6.8/10 He's like a laiden truck. He's slow, but if you're in his way he'll hit you like, well, a truck.
Metacharacteristic Value: .58
Ku'gath Plague Father
Strengths
+1 W over a GUO
Makes Nurglings better, Can't be charged reliably.
Immortal Commander is a very apt warlord trait. Shame he won't get iron arm 5/6 games.
Weaknesses
Everything bad about a GUO, but not ML3.
Things To Watch
Necrotic Missiles can sweep the floor for you against the right targets.
Price 5/10 Cheaper than a GUO, but not to make him reliable.
Survivability 9/10 Poor Access to Iron Arm, no greater rewards.
Damage Output 6/10 Read: GUO, then add one attack.
Rules 6/10 Force Multiplier for... Nurglings.
Army Bonuses 7/10 Read: GUO, then add one wound.
Overall 6.6/10 He's arguably just a GUO with less options and a name.
Metacharacteristic Value: .60
Slaanesh
Keeper of Secrets
Strengths
Speedy for a normal Monster.
Can dent AV14.
Rolls on Telepathy, Excess can actually be pretty cool. Acquiesence can set up awesome charges and keep your side alive.
Second only to the Thirster in Close Combat.
Fleet, D6+3" runs.
Weaknesses
Not FMC.
Things To Watch
Your Walking monster sprinting across the board at pace with your FMCs.
Standard Loadout
ML 3 with Greater/Greater/Lesser Reward.
Lash of Despair/witstealer sword can be a very potent combination. Also makes excellent use of the Gloryseeker blade, if you decide to go exalted. Lash is equivalent to charge range, so usually softens his target with it before charging.
Price 7/10 His Pricelessness is mitigated by his lack of wings.
Survivability 5/10 thirsters have a 3+. GUOs have their Toughness. LOC have re-rolls of 1. Slaanesh just has speed. Speed doesn't keep you alive if you're not a tank.
Damage Output 7/10 Pretty gory in close combat. Can have a decent amount of anti-infantry/light vehicle shooting.
Rules 6/10 the D6+3" is pretty nice, as is fleet.
Army Bonuses 5/10 He doesn't add to the army, unless you want Slaaneshi DP.
Overall 6/10 About the same as a thirster, the speed makes up for the lack of wings.
Metacharacteristic Value: .72
Afterword
Winner:
Our winner is, and always will be fateweaver. Are you surprised? Didn't think so.
The Wooden Spoon
While all the Greater Daemons are killy and generally pretty good in the right place, our loser is Skarbrand. Skarbrand would be awesome in a marine army, and would be perfect for a Blood Angels army. Unfortunately, he supports Khorne. Khorne is already killy without him.