"An American Werewolf in London" (1981): A classic in the funny scary movie category. The special effects for David turning into a werewolf were gut-wrenching at the time, and they still hold up (the film also won an Oscar for Best Makeup). The admonition from the denizens of a local pub to stay on the path, and the realization of the two travelers that they are no longer on the path, is one of the best moments in horror history. One of the funniest? When David is confronted by some of his victims in a movie theater and politely urged to kill himself.
"Shaun of the Dead" (2004): Another classic, this stands with "Werewolf" at the pinnacle of comedy horror films. It's got a great hero in Shaun, a ne'er do well forced into leading his friends through a zombie outbreak while trying to win his girlfriend back. There's a perfect mix of contrasting ambitions -- impress the lady, don't get eaten by the shambling hordes. And it's tough to beat a zombie fight scene choreographed to Queen's "Don't Stop Me Now" playing on the jukebox.
"Zombieland" (2009): This is the newest addition to my list. The opening sequence is an entertaining mix of gross-out horror humor. It shows the tipping point in the zombie apocalypse, just before the zombies have taken over completely, when people are still out on the streets or trying to do their jobs, while the narrator ticks off a list of rules for surviving zombies. Also featured: some great gags (including a wonderful cameo I won't ruin for you), a few startling moments, and an epic stand-off at the end.
"Evil Dead II" (1987): This trilogy got more and more campy as it went on. The original was close to a straight horror flick. But the sequel/remake/ of "The Evil Dead" is the one that best balances the humor and horror. Writer/Director Sam Raimi cemented his influential visual style here, with the creepy point of view shots and a slapstick quality to the action. "Evil Dead II" also has Ash vs. an army of little Ashes, and Ash vs. his own hand, which leads to him replacing it with a chainsaw. Say it with me -- "Dead by dawn! Dead by dawn!"
"The Return of the Living Dead" (1985): Very campy 80s flick that popularized the notion of punk zombies. Yes, that's a zombie with a mohawk on the movie poster, a mohawk even though there is no hair attached to it. It's also the first zombie movie I remember where the zombies chanted "Brains!" although it may not have been the very first to do that. Military conspiracy theories (complete with a sort of anti-disclaimer at the beginning that the events all happened and all the names are real), Clu Gulager fighting with a naked zombie, and a graveyard party. Plus, the split dog scene always makes me cry a little.