My most used combo was Garrus and Liara for two reasons I play as a solider so Garrus makes up for my low skills in tech and Liara makes up for my lack of biotics.
Ashley and Liara. So i have something hawt to look at in the elevators. My teammates were never really useful anyways.
Ashley, Liara and Garrus. I play with Ashley on my Bionic, and Liara on my Soldier, and Garrus has never left my side... But yet is always the first to die.
Garrus was really only useful in combat if you maxed out his sniper ability and gave him High Explosive Rounds. Every 10 seconds something would just get shot into firey dust. I also really liked him as a character, so I just enjoyed having him with me on missions so I could hear his comments.
I used Garrus because of his character, as well. He's probably one of the cooler characters in the game.
I also had Wrex in my party a lot, just because I liked to see the tension between him and Garrus, seeing as how the Turians deployed the virus that virtually wiped out the Krogans.
I use Wrex and Ashley the most. If I use any one else, it's Garrus. Does anyone use Kaiden or Tali? I don't think in any of my playthroughs have I had either of them in my squad for more than one mission/landing total. They just aren't useful in my opinion.
I've used almost all combinations in many playthroughs... that said, I have to propose that both Garrus and Wrex were my favorites.
Since Mass Effect 2 is coming out soon, I need to get the game again and replay it more so I can level up my character but the combo I always went with was Kaiden and Ashley as my teammates, It's a shame that one of them had to die but I kept Ashley since she was my love interest and I didn't want to lose her. lol. <3 After Kaiden died I went with either Tali or Liara on my team and switched depending on what I needed.