Tough choice. I'd go for Mordin or Garrus. Zaeed is cool as well but he doesn't even have a dialogue wheel so its hard to know what he really is like with just him talking about his war stories.
Only one? *makes sad puppy face* But I like them all! Except Jacob (who I actually like fine except that fem!Shep gets all scary around him)...
But if I can only pick one, then it would be Garrus. He and Tali are the two that my Shep trusted the most to watch her back with Cerberus around, and really the open armed "Garrus!" greeting that Shepard gives him was just perfectly how I felt at seeing him again.
I just figure that he's been in the first two games and is more or less the main stable 'alien sidekick' of the story so far, with that in mind he has more of a right to return than the others.
Hell, he was on the cover of the first game.... but then again so was Ashley and look at the crummy hand Bioware gave her in the sequel.
That said, if Ashley and Wrex want to show up as well, I would welcome them warmly.