playing Mass Effect 2 for the second time and beating it on PS3(having already played and beaten ME2 on my 360 twice as well) I've noticed that whenever I use Garrus and Thane shit gets done quick, specailly after getting all the upgrades(weapons and shielding) and their loyalty powers, I hardly even need to command them to use their powers.(maybe a mix if good AI and an overall skill set)
other good combinations in my opinion:(on a side note: I've used the soldier class for all my playthroughs)
Miranda&Jack
Miranda&Jacob
Garrus&Grunt
Katsumi&Legion
Thane&Katsumi
Worst team:
Mordin&Samara(they just always get mowed down by practically every enemy)
Zaeed in general( I find is techniques to be quite ineffective...maybe I'm not using him right)
you don't have to limit yourselves to just Mass Effect 2(though PS3 owners will)