Rhaomi: I'm pretty sure jefgodesky's substantive comment should rank higher -- if you scroll down a little afterwards, he mentions having hit the character limit (there's a character limit?) and picks up where he left off for a few more screens.
I have the (dubious?) honor of the longest answer on Ask MetaFilter. I posted the answer in 2005 and at the time it turned out that MetaFilter clipped comments to ~32K when displaying them (but the full answer was placed in the database and currently renders correctly).