Almost all of the cast in Mass Effect found their way into my heart. In fact I honestly think the only reason I want to play Mass Effect 3 is to see resolution with beloved characters, to rekindle the romance I had with Ashley in Mass Effect 1, to have Garrus fight alongside me one last time.
These days I find the plot becoming more and more ridiculous, I couldn't care less about gameplay additions and I've never read any of the books. And yet, wanting to once again romance Ashley and fight alongside Wrex and Garrus seems to be enough for me now.
For the ones with which I have the most affection:
Ashley, Wrex, Garrus, Jack, Mordin, Grunt, Legion, Samara, Thane and Tali
Also while I'm here, I love myself some Arbiter from Halo 2 and 3.