1. Silver Streak
Gene Wilder and Richard Pryor's first pairing: racial-barrier milestone, and damn funny. Go re-watch the scene where Pryor teaches Wilder to "walk black." Also, Wilder introduces the concept of Transcendent Blackface, a.k.a. blackface that manages to be such a perfect mix of racial critique and pants-peeing hilarity that everybody wins, and nobody (well, almost) is offended. (See: Downey Jr., Robert, Tropic Thunder.)
2. Butch Cassidy and the Sundance Kid
The western-outlaw story and script are aces, but fact is Robert Redford and Paul Newman could have just crank-called Domino's for two hours and we'd still pay to watch them. Bantering and bickering like Cary Grant and Rosalind Russell, they have the kind of chemistry real-life married couples can only dream of.
3. 48 Hours
Hey, remember when Nick Nolte wasn't call-the-cops crazy? He was the perfect crusty straight man to Eddie Murphy's careening, wisecracking criminal. Also, the soundtrack is like bathing in essence of eighties.
4. The Blues Brothers
Before Jake and Elwood Blues were the most popular Halloween costume of all time, they were at the center of what may be the only great buddy musical (and movie based on an SNL skit) ever. John Belushi and Dan Aykroyd are true forefathers to the Apatow School in that they seem to be goofing off between takes even when the camera's rolling, and it works. Also: the Ray Charles scene. Genius.
5. Midnight Run
Robert DeNiro is a brash bounty hunter. Charles Grodin is his soft-spoken prisoner/accountant. Grodin's touchy-feely. DeNiro doesn't do touchy-feely. In 1988, when this came out, DeNiro was only known for brooding or menacing roles -- Raging Bull, Angel Heart -- and this was arguably his first comedic turn, a.k.a. the movie that birthed Stock Funny DeNiro, the character he's still doing 20 years on. It was funnier in 1988.