First I would like to list the units I think will be the best choices in 6th edition for the style of competitive play I invision:
HQ
— Hive Tyrant
I plan to run a pair. One Tyrant will have wings while the other will have an armored shell (2+ armor save) with a pair of Tyrant Guard. The Flyrant will pick his psychic powers from the Biomancy lore while the walking Tyrant will use Paroxsym, Leech Essence plus Hive Commander and Old Adversary. Both will have twin linked devourers with Brainleech ammo plus bonesword and lash whip (mainly for the free lash whip). The role of the Flyrant is to act as either a vanguard or hunt down enemy fliers. The role of the walking Tyrant is to act as the main synaptic hub for the horde. Hordes are back in as competitive for a host of reasons such as Fearless units no longer suffers from No Retreat armor saves. Of course both Tyrants will have toxin sacs because poison is more powerful now since you roll to wound as normal.
Elites
— Yrmgarl Genestealers
Yrmgarl genestealers are one of the few units in 40k that can still assault the turn they arrive from reserve and that's worth taking advantage of in my opinion. You'll need a large brood of eight or more now due to the new Snap Fire rule. Morph up to T5 on the charge to shrug off bolters and lasguns. They can easily destroy vehicles and dreadnaughts on the charge now due to hull points or tie down heavy support in protracted melee such as Long Fangs. Shooting is still the king now so any enemy unit you can prevent from shooting for a turn or more can really help a lot to sway the battle in your favor.
— Hive Guard
Hive Guard have always been quite awesome and even more so now since they ignore the new Night Fight rules. I plan to run two full broods of three each.
Troops
— Tervigon
I know if you follow my blog you are aware I've never been a big fan of the Tervigon but because I want to incorporate more of a horde element in my army she is in now. I will probably stick with her normal psychic powers since they are great for hordes. She can outflank due to the walking Tyrant's Hive Commander which is very tactical in the right situations. Basically the Tervigon buffs the army and can poop out more scoring units when necessary. I believe you need at least four scoring units to be competitive.
— Devilgants
Better than ever and I plan to run a big brood with toxin sacs. They will be an excellent anti troop shooting unit gaining Preferred Enemy from the Walking Tyrant and can bring down some enemy units in melee due to poison.
— Genestealers
I am planning to run one brood of ten lead by a Broodlord (Biomancy) and they will also have toxin sacs. They can play a defensive role protecting the hub of your army, or infiltrate/outflank in the right situations. Genestealers are still one of the premiere melee units and the rending will help a lot versus the new 2+ armor save.
Fast Attack
— Shrikes
I plan to run mine with boneswords, lash whips and toxin sacs. I'd like to run a brood of five. This is another premiere melee unit that also ignores the 2+ armor save and is very fast. They can deep strike if necessary and are a big threat. If you want to save a few points then Raveners are a good substitute but will struggle more versus units such as terminators since they can only rend while the Shrikes with boneswords outright ignore armor saves and reroll failed wounds due to poison versus T4 and lower. Raveners can't take any upgrades and really need the Broodlord to buff them.
So there are my top choices. Many are the same as before and there are a few new units. Null deployment is no longer possible with the new rules but on the plus side you're probably not going to see as much mech out there initially except for armies like Necron Nightscythe spam (yuck... the new MSU mech army). A challenge for all Tyranid players is how you will effectively deal with enemy fliers. You'll notice I dropped the Tyrannofex in favor of a Tervigon. I do not think the Tyrannofex is as strong of a choice now and does not greatly benefit from the walking Tyrant's Preferred Enemy versus armor... You're just rerolling 1s to hit with only two shots. Remember that fliers can neither control objectives nor contest them so it will be all about positional play to force them into range of the Hive Guard who'll greatly benefit from the walking Tyrant's Preferred Enemy since they can generate up to six shots per brood.
Here is a sample 2000 point army list:
Hive Tyrant - wings, twin linked devourer w. BL ammo, bonesword & lash whip, toxin sacs, Biomancy
Hive Tyrant - armored shell, twin linked devourer w. BL ammo, bonesword & lash whip, toxin sacs, Hive Commander, Old Adversary
2x Tyrant Guard w. boneswords
3x Hive Guard
3x Hive Guard
8x Yrmgarl genestealer
15x Devilgant w. toxin sacs
Tervigon - toxin sacs, Dominion, Catalyst & Onslaught
9x Genestealer w. toxin sacs
- Broodlord (Biomancy)
5x Shrike - toxin sacs, boneswords & lash whips
I really like this list a lot. Of course I've got a good deal of work ahead to build and paint all the new models. Probably within a month or so I'll be ready to start playing my new brood. w00t !!