IMO, it's a tight race between Vasari and TEC.  Here's what I like about them:
 
VASARI:
*  The phase lane you get with the Kosutra shot is a game changer. (You can jump past the front line without having to fight your way thru).
*  The Skiranta (Repair Cloud) gives 30 hull per sec on infinite targets for a total of 300 hull per ship [including itself].
*  The Marauder's (Phase Out Hull) interrupt's w/7000 range, and it's a mobile phase lane generator.
*  Planet killing machine: Jarrasul (Drain Planet), Vulkoras (Siege Platform w/Assault Specialization), and then Skirtana (Replicate Forces: Used on Siege Platforms)
*  Starbase: Mobile, can make a Phase Lane, has 4 weapon upgrades (+20% damage), can self heal from debris, frontal shield, trade port + colony pods.
 
TEC:
* Economic Superpower.
* Marza (Missle Barrage). Nuff said
* Novalith on auto-fire.  Nuff said.  2 shots and it put's a planet to sleep immediately (-100% pop, and uncolonizable for a long time).  If you build 4 or more of these things, you can put an entire star system to sleep.
* Starbase:  The big red button + Trade Ports + Frigate Factory = Nuff said.
* Eats strikecraft: Dunov (Magnetize), Kol (Flak Burst)