Fawkes. he kills deathclaws in seconds and he never dies. he makes the game boring though.
for fallout new vegas, i think Boone, although sometimes his sniper training is a little annoying when he steals my kills >:
Dogmeat, hands down.
His ability to pick up on enemies (sometimes even a cell away) and turn in the direction they are in makes him indispensable for sneaking sniping players. He's also saved me from critters sneaking up on me countless times. People say he makes sneaking impossible, but that is not so. His barking does not tip off enemies and actually following his lead opens up a lot of one shot kill opportunities. Also his post Broken Steel healh makes him a juggernaut of destruction, but he is NOT immortal and does need to be looked after to some degree, so he does not make the game easy and boring the way Fawkes does.
He's such a good boy. : )