The single funniest work ever published in English.
This collection of Sam and Max's adventures is now out of print, and very difficult to find. But you should try, anyway. Because you'll laugh, hard.
This book is one of the few things that's caused Spontaneous Laughter in me. I'm serious, it makes me laugh when I'm not even reading it. It's that funny.
In other news, the funniest Japanese Comic ever written is "What's Michael?", available in english from Studio Proteus.