I voted for Mech Fists 'cause in the later game you need the extra power, but I love Love & Hate for the 2-fisted animations! In the 6-man at a time fights
a Ballistic Fist is rockin' fun.
Also, Pushy makes an extra electric zap kind of a flash that looks great. I like to give that to Veronica just to watch her blast away.
I haven't built a character with enough Strength to swing a bumper, but it looks awesome, too. Hurts to get hit with, I do know that.