The Best Crossovers Ever

The upcoming crossover episode of The Simpsons and Family Guy has a lot of fans excited.  Certain characters seem destined to eventually meet. Most crossovers are just guilty pleasures, nothing more than glorified fan-fiction, but it all still manages to entertain, even if in the end it's all pretty silly.

I'm not counting spin-offs or gimmicky cameos, and I'm not considering ones that are already in a shared universe (such as C.S.I. or Marvel's The Avengers) I'm only talking about major crossovers.  Here are the ones that come to mind:

Alien vs. Pedator -- It's not great, but it's not as bad as I expected. I enjoyed both franchises, so blending them together didn't strain logic too much. The comics managed to have better stories than the movies, though.

Frankenstein Meets the Wolf Man -- This was such a brilliant idea that it launched an entire series of movies combining various Universal monsters.

Freddy vs. Jason -- This was probably one of the worst movies I've ever seen, but the concept of the two slasher maniacs colliding was intriguing, even if the execution left a lot to be desired.

The Jetsons Meet the Flintstones -- One was set in the future and the other in the prehistoric past, yet they managed to come together.

King Kong vs. Godzilla -- The execution was terrible, but mashing up giant monster movies is a no-brainer.

Scooby-Doo Meets Batman -- It just didn't make sense, but Warner Bros. owned both properties and they both solved crimes and mysteries, so why not?

What do you think?