Mostly "The only thing that is impossible, is something being impossible." That got me into a serious fight with my mathematics teacher who was trying to force the belief that things can be impossible onto my class.
But just to annoy my friend whenever I beat him in chess or an RTS I often say:
"There are always alternatives to fighting, you just suck at them all."
Also when I do well on a test:
"I am not gifted, smart or blessed by the gods, you just suck so in comparison I am better."
Man, reflecting on my motto's, I think I might be a jerk. Oh well.