Every god damn theme from Monkey Island is great, but i especially love the Le Chuck's one. I've thought about somehow coering it a thousand time and always got way too lazy, but god, this has to be one of the greatest theme ever.
I actually love most of those old lucasart themes (sam and max had a lot of awesome stuff, and the ennio morricone rip off from outlaw was pretty well done), especially in those old "crapy" MIDI versions. Usually hate MIDI but I dunno, this crap sound just seems to fit them so well.
Drink Cheap, Evil-Smellin Beer Everyday of Your Life.