Cracked.com has a nice list of The 10 Most Irreplaceable Performances in Comedy Films. I was going to pass it by and not post, until I cam across #6, Alan RIckman in SpaceQuest:

Alan Rickman can pack so much tragedy, rage, injustice and disgust into a single line that you'd swear it was a McRib.

I'd have to give it to him for his performance as Marvin in The Hitchhiker's Guide to the Galaxy... every time Marvin said anything with Rickman's voice I burst out laughing.