I looked at various rankings released by sites like IMDB, ranker, Rolling Stone, and others to come up with a "composite" list of top comedies of all time. Since I really only looked at the top 10 of their lists, I'm only going to include the top 10 here. It's interesting to note that not a single movie was ranked in the top 10 by all of the various sites meaning that the question is VERY MUCH open to debate, but here is what the "consensus" would be:

1. Airplane
2. Monty Python and the Holy Grail
3. Blazing Saddles
4. Caddyshack
5. Spinal Tap
6. Animal House
7. Young Frankenstein
8. Anchorman
9. Life of Brian
10. The Hangover

The Jerk, Groundhog Day, Dumb and Dumber, Borat, and Ghostbusters are honerable mentions as they were all included in top 10 by multiple sites but not ranked high enough to crack overall top 10.