Since I only collect Tyranids all other units on this list are just based on the fact that I think they look cool.
IG: Cadian Kasrkin - I'd say the most futuristic infrantry in the game
Dark Eldar: Razorwing Jetfighter - you must admit that is one of the coolest flyers
Necrons: Most of the Necron units are cool, but the best is Catacomb Command Barge- that is the way you should arrive to a battle, a true command unit
Space Marines: Scout Snipers - I actually bought these just to paint them.
Then to Tyranids:
Tyranids: Warriors My current army, I have a load of these and a prime. The fluff and models are great and they're a unit that has divided opinion for a long time. Perhaps my favorite change in the new dex was bringing back mixed weapon units so I can model a prime and field it as a regular warrior with a sword/lash and talons to give some CC ability. A minimum sized unit with a barbed strangler seems viable enough for competitive play too. Cheap synapse, scoring and an assault large blast weapon seems like a decent synapse choice given the options
I must say I share your love for the Warriors. But the coolest and my favourite is the Swarmlord - I would have no other unit leading my army.