Barring TA, my favorite in DA is the Thalans. My favorite in DL is the Yor.
In DL, the Yor are awesome for one single reason, 25% miniaturization bonus. It just makes everything better, faster colonizers, bigger warships, etc. They also get really good starting techs.
The Thalans are amazing in DA. With their super hive ability, it makes building planets quick, cheap, and easy. That's something that helps through the whole game unlike other super abilites that only help during certain phases of the game. Also, they have fantastic starting bonuses. Not only do they get 33% in social and military production for free, but they have a 10% miniaturization bonus to boot. And don't forget about that class 15 homeworld. The Yor are still pretty good in DA, the super isolation ability helps fend off colonizers and transports, but they're not as powerful as they were in DL. They would be my second choice in DA. I like Custom Race, because of the control over all aspects, but they just don't get perks the way the majors do. I don't play custom much anymore, but I still like it a lot.
I can understand while people like the Terrans. For one, it's nice to be human, but otherwise, they have very strong diplomacy which is good for less aggressive games. When I first started playing GC2, I played a lot of games with good alignment as the Terrans because I was always trying to avoid war. However, now I spend most of the game at war with most of the races so diplomacy for the sake of avoiding war isn't much use to me. My philosphy now is "yea, bring it on baby".