For my calculative, pr-minded (paragon conversation options, but few, but severe renegade actions, like killing the council, he can smooth-talk himself out of the worst situations) sentinel with mind dominate, I pick Miranda. She is smart, connected and ressourceful, but weak-willed and controllable. (Plus great genes for the romance, gotta have offspring for that future empire)
My forceful and aggressive socialist paragon would pick Samara, she matches his fight against social injustice and wrongs. (He is my me-ish character)
Then there is my **** Shepard, he is a loner and would probably not want a second in command. If he had to pick one, he would probably pick Zaeed or Jack, they are both crazy and won't ever question his renegade actions. Plus their shiftiness and the possibility of them betraying him excites him.