I know a lot of people here are going to say either Garrus or Tali, but I say Mordin was by far the most interesting and complex party member. Garrus is my homeboy and Tali is a sweetie, but Mordin is just fascinating. His dealings with the genophage and the guilt he has from it shows how honorable he is, but he will do whatever it takes to ensure the greater good. His loyalty mission has a complex moral conundrum and you can see Mordin's rational reasoning for the use of the genophage. He may look like a simple comic relief character at first, but he becomes more unique than any other squad member once you get to know more about him. Mordin has by far, the most depth than any of the squad members combined. I say Wrex is a close second to Mordin, along with Legion and Thane.
I almost forgot to mention, he does Gilbert and fucking Sullivan!