And Tyranids 
Seconded. Tyranids are probably the single best race the Evil Empire have pieced together out of various scifi staples.
From Genestealer Cults to the full blown Hive Fleets they have so many different facets that I'd quite happily include them in any scifi setting. Also as the models aren't humanoid they don't suffer from the Heroic Scale mishapen style of so many of the EEs range so you can put any companies models up against them.
I'm not to keen on some of the slight changes to their background recently, but that's nothing out of the ordinary, not content with their less than perfect game they have begun to erode their only real strength, the back story and characters. Which is sad because while I have pretty much given up on the company model and rules wise after some 15 years the background is still something which holds a lot in my heart and mind.