I have a full on 1000ish point army of Tyranids.
HQ:
Hive Tyrant
Warrior Brood
Tyranid Prime
Troops:
Termagants 7
Termagants 7
Termagants 6
Hormagaunts 6
Elites:
Genestealer Brood w/ Broodlord 10
Heavy Support:
Tyrannofex with Acid Spray
The Tyrannofex is nearly invincible when facing my friend's Space Marines, as he neglected to get more heavy weapons. He has a plasma gun, a flamer, a missile launcher, and a heavy bolter, but he always spreads them out. When a normal marine has to roll a 3+ then a 6, and I can armor save with a 2+, they're probably not going to kill it. Especially when you give it regeneration, so it can recover up to ALL of it's lost wounds in a single turn.
The Hive Tyrant is far more killable, but far more kill-y as well. The Tyrannofex has to roll 6/2 to cause a wound, but the Hive Tyrant only has to roll a 2/3 and the boneswords can cause instant death.
The Warriors are great for... Well, actually they kind of suck, but I'm trying to work out some good tactics for them.
Swarm of gribblies: Check.
Swarm of elite gribblies: Check
Big nasty badass monsters with prohibitive point cost: Check
Also, the Tyrannofex can fire one weapon that uses normal flamer template rules, then another that is a ranged flamer template that can be tilted and such to represent a spray of acid. AND it has a four shot burst gun built into it's body. Against guardsmen, it would be an utterly rape-tastic god of battle. Against Space Marines, it has merely become the most annoyingly tough to kill bastard on the field.