My ME1 squads usually depended on my Shepard's class, but my favorite trio was my Vanguard Shepard, Ashley, and Garrus. My Infiltrator Shepard with Wrex and Kaidan was pretty good, too. But Ash, Garrus, and Wrex were by far my favorite squadmates.
ME2 was a little different because I didn't need to worry as much about making a balanced squad, but my primary lineup for my main Shepard (the Vanguard) was Miranda (mostly because of her evolved abilities, plus Overload and Warp) and Garrus. Garrus, Grunt, and Mordin were my favorite of the ME2 squadmates.