For me thus far, it's got to be the Vasari. I love the way the tech tree is set up and the weapons available to them, much more powerful and useful in my opinion then those used by the TEC, who tend to be too economy based for my like.

The ability to build in with these guys is great, as is their phase jump abilities -- no one has any idea how happy I was the first time I crossed a system with over fifty planets in a fraction of the time it would normally have taken me thanks to their advanced Phasing technology, especially useful for the larger empires.

I always thought that the TEC ships seemed sort of boxy and awkward compared to the Vasari or Advent ships. =D

A while back, when I was playing as the Vasari I noticed how awesome their capital ship weapons looked. Outside of that, the Advent definitely have some amazing ship designs, although personally I find the Vasari ship designs equally as interesting.