1940s:
Arsenic and Old Lace
1950s:
Some Like It Hot
1960s:
Dr. Strangelove
(Runner-up: The Odd Couple)
1970s:
Monty Python and the Holy Grail
(Runner-up: Young Frankenstein)
1980s:
A Christmas Story
1990s:
The Big Lebowski
(Runner-up: What About Bob?)
2000s:
Mr. Bean's Holiday
The Out of Towners - with Jack Lemmon and Sandy Dennis. Not the remake.
Clueless - just something about that movie that cracks me up.
Drop Dead Gorgeous is a hoot too.