Hmmm, that's a tough one. And I don't mean because I can't use Family Guy.
One of the best lines of all time is "I didn't ask" from Batman. The way Nicholson delivered it made it an instant classic in my mind. I wish I remembered to use it more.
However, that line needs to be delivered for its brilliance to be realized. Thus, probably the greatest quote that goes both ways: delivered and written ("heh, like a bisexual" SORRY THAT WAS FROM FAMILY GUY) is:
"You fell victim to one of the classic blunders. The most famous of which is never get involved in a land war in Asia. But only slightly less well known is this: never go in against a Cicilian when death is on the line."
It may take a few times through to realize the idea behind "never get involved in a land war in Asia," but after it clicks, it is sheer brilliance. If you forced me to pick one, that would probably be it. But there are so many, it is hard to pick. AFI recently did a list of the 100 greatest movie quotes and there weren't any from The Princess Bride. I'm sorry. TPB is one of the most brilliantly written movies ever. To validate a list such as AFI's, at least one quote from that movie had to be included.