Garrus Vakarian.
Why? Because he's Garrus Vakarian. If I'm going on an epic adventure, I need someone that can watch my back. He's helped save the universe from annihiliation twice. Those are some pretty solid credentials. Besides, he endeared himself to me early on in the original
Mass Effect when his solution to resolving a hostage situation was to headshot the hostage taker. He seems to have the kind of personality and personal philosophy I could get on with well too, willing to bend or break the rules if it's necessary to do so to do the right thing.