I would say that old Commander Sheppard is barred from this conversation since you control so much of his conversation, he is a representation of yourself (unless you are playing trying to get moral achievements). That leaves several to choose from, the giant Wrex? Your homeboy Garrus? Maybe a girlfriend in Tali or Liara? Are you really weird and liked Ashley or Kiaden the best?
My favorite is easily Garrus. He is calm cool and collective in every position, and even shoots his sniper from the hip in the battles, what an amazing shot. Wrex is a good bro, but he is dumb and stuck in the past. Tali and Liara are both to needy and Kiaden and Ashley are both annoying and needy as well. Garrus is just the man, the Turian Sniper is just as important to the Mass Effect franchise as Commander Sheppard.