For when somebody does something stupid:
Personally I'd (insert advice here), but maybe that's just me, I don't like (insert consequence here).
For example I'm moving some very heavy stock through a small aisle and when asked to move some idiot customer flattens themself up against the shelves, hence "Personally I'd get out of the way, but maybe that's just me, I don't like getting crushed into shelving by passing boxes of furniture..." which was followed by a snide "thank you" when they moved.
A: Fuck you
B: Get in line.
A: You're stupid.
B: Yes, evidently I'm the stupid one. (takes about five seconds to sink in)
A: You failed at your mom last night.
B: Then teach me oh masterful motherfucker.
A: Your face makes me bleed.
B: For reference, it's called a head-butt.
A: Your mom is so fat she falls of both sides of the bed.
B: Dude, my mum has cancer, are you proud of yourself now?
("Your mom" jokes really don't work on me)
A: Go back to space.
B: No-no, I have to probe you first.
A: You suck.
B: You blow, I should know.
A: You're a fag.
B: In your dreams.
A: No you stupid!
B: How ironic.
A: You're such a wierdo.
B: Thank you, I'm sure one day you'll be interesting too.
A: That doesn't make any sense.
B: I'm sorry, let me dumb it down for you.
A: Speak proper English.
B: How would you know?
Or
A: You're too stupid to understand this question.
B: Pardon? (ignore hir)
A: Shut up.
B: Ladies first.