THere have been some good ones mentioned but I have a few more to offer.
My absolute fave is "The Out-of-Towners" (1970) with Jack Lemmon and Sandy Dennis-it always has me in stitches as if I were watching for the first time all over again.
Another fave is "Gone Fishin" with Danny Glover and Joe Pesci-those 2 together are hilarious!
I also love "Planes, Trains and Automobiles" with Steve Martin and John Candy-another great combo there.
"Dr. Strangelove" is a must see as well-I haven't seen it in a long time but Peter Sellers is great in that!
Nikita