I looked at all three games and to me there is only one answer to this question, the illusive man. To me he seems like a very well thought out character, and by that I mean I get why he does what he does, it’s not just random to drive the story, which it does, but it does so in a way that makes sense. I got the four mass effect novels for Christmas and have started reading them and so far I’m very impressed and can’t wait till I get to the part about the illusive man.
I loved him as a character because he wasn't a fighting type guy, his real weapon was mind and I liked that, and he was trying to do the right thing just using the wrong way to go about it, but to be fair, the entire galaxy was about to be under siege by sentient machines, so I feel like it was warranted for someone like him, who wanted to protect humanity at whatever cost.
That’s really it, other characters that came close were Anderson, Garrus and Tali. But I just like the illusive man so much and it really is a fantastic creation by the bioware team. Plus he has the best theme than any other character. Thanks for reading this, if you have any comments then write them down and I’ll reply as quickly as I can, thanks.