damn that's hard, so to make easier I'll state my top five
in no particular order
*Skua September- a character from Alan Dean Foster's commonwealth series; a giant of a man, with prematurely white hair, a thousand ways to kill you, and a sense of humor under fire that puts John McClane to shame
*Brian Duffy- the main character in the Drawing of the Dark (look it up, it's one of the best entertaining reads nobody has ever heard of): a drunken Irish Landsknecht, who spends much of his time kicking ass and getting shitfaced then kicking more ass, also he is the reincarnation of king Aurthur
*Bean- from the Ender's series, wickedly intelligent, and "badass-itude" in much higher degrees than even Ender could hope to achieve
*Drizz't- of course who couldn't mention him, though he does get waaaaaaaay to soft later on
*Elric of Melnebone'- not the most heroic or epic of the lot, but none the less fairly awesome; soul-sucking sword and all
NoMoreSanity said:
Rorschach from Watchmen. He is the definition of badass.
"comic-book" characters don't really count as book characters