Depends on your sense of humour, but the following are a MUST:
Virtually any of the Marx Brothers' films, but would particularly recommend A Night at the Opera;
Some Like it Hot
The Producers (Zero Mostel and Gene Wilder)
Blazing Saddles
And, for British humour:
Kind Hearts and Coronets
The Ladykillers (the Alaistair Sim version, not the remake)
The Belles of St Trinian's (don't bother about any of the other St Trinian's films, they're not nearly so good)
I'm all right, Jack (biting satire, and Peter Sellers at his best, not to mention the other all-star comedy cast)
(Another Peter Sellers film I would highly recommend is a little-known gem called The Naked Truth, about a gang of celebs who set out to nobble the publisher of a scandal mag - highly topical today, though it was made over 50 years ago!)
That should keep you going until Christmas!