The best medicine? No. Any sort of medicine at all? Well, that one is at least debatable.
It's hard to get funding for work that examines the health effects of humor, but there are some studies out there that show regularly laughing out loud—not just quietly enjoying something funny—can have real benefits.
Laughing, among other things, releases endorphins; these pleasure-inducing neurotransmitters set off a chain of reactions that result in your blood vessels widening—and wider blood vessels are good for a host of reasons. A 2005 study supported this: Participants showed an average 22 percent increased blood flow after having watched a comedy (sad movies, on the other hand, resulted in contracting blood vessels).
Laughter has also been shown to burn calories, reduce stress, and improve mood, although it's hard to imagine laughter —or at least the situations that cause it—failing to do the latter two. But even if the health benefits are obvious and minimal, it can't hurt to go out and enjoy yourself.