In terms of Gameplay, I guess I have to do it on average. Liara was easily the best for me in ME1, because once she had proper armor and her talents were spec'd up, she was a biotic juggernaut, however, Garrus was a good all-around character in ME1 too, having some light tech powers as well as the ability to use sniper and assault rifles, and then he became my solid right-hand-man in both ME2 and 3, so gameplay would be Garrus.
Story is a toss-up between Mordin and Javik. The bulk of Mordin's best is in ME2, but as a whole, his dialogue is the one that sticks with me above all others. Javik, however, does have some good lines that do make me genuinely feel for this character, but I guess the winner is Mordin for his ties to what i felt to be the best major arc in the series.