Random Comebacks
Dude: YO MAMA!
Me: YO PAPA!

Dude: Your face...
Me: I'd rather have no face than to have one like yours. :)

(Back in the olden days)
Dude: You're so ugly!
Me: I can have plastic surgery; but for you that problem's unfixable.

Random Insults
1) You, sir, are a man of rare intelligence... It's rare that you show any.

2) If laughter were really the best medicine, your face would be curing the world.

3) I truly love what you've done with your hair. How did you get it to come out of your nostrils like that?