The Yor.
I really like their background lore and general attitude. "Sure, we'll make an alliance with you, but you are still dead meat once you're no longer of use to us. Nothing personal."
Their Collectives have the highest output of all factories, and they've got the best farms in GalCiv 2 (Charging Stalks). Their economy is also unique. Instead of spamming Stock Markets, like the other races, you need to spam Charging Stalks.
Overall, I love how their playstyle is so differently compared to the other races.
The only downside of playing the Yor is, that their economy, while unique, is also pretty weak. Which isn't exactly helped by the high maintenance costs of their manufacturing improvements. You really need to micro them a lot, in order to get the most out of the Yor. Which suits me fine. I like to micro.