You should always choose characters for the squad of each mission based on your preferences, both in terms of which have skills that mesh well with Commander Shepard's, but also in terms of who you like most on a personal level. The answer, obviously, is deeply subjective.
Mass Effect 3 brings together a slightly smaller squad of warriors to take the fight to the Reapers, but there's enough of them that you'll always be leaving some folks behind on the Normandy - which always begs the question - what's the best squad for each mission in ME3?