If you were a fan of Rudolph the Red-Nosed Reindeer growing up (or if you still are), then you might like this film. Made with the same stop-motion animation by Rankin/Bass, Mad Monster Party? is more suited for Halloween than Christmas, but it's still watchable light-hearted entertainment. At the venerable age of 80, Boris Karloff is the big name in the film, giving voice to Baron Boris von Frankenstein. After perfecting a deadly explosive potion, the baron invites all the members of the monster society to his Island of Evil to celebrate. Unbeknownst to them, he is also planning to announce his retirement as head of the monster society, handing his research over to his geeky pharmacist nephew, Felix Flankin (Allen Swift, who played several other voices as well). Felix is unaware of his pending inheritance, and of most of what his uncle does as well, so he sets off innocently for the Island of Evil to attend the party. Once there he runs afoul of Francesca (Gale Garnett), Baron Frankenstein's voluptuous assistant. She wants to take over the research, and sees Felix as an unwanted rival.
Almost every monster in cinema appears in the movie: Dracula, the Mummy, Frankenstein's Monster, his mate (voiced by Phyllis Diller, whose grating laughs got tiresome after a while), and more. Baron Frankenstein even has a private army of zombies to do his bidding. The plot is pretty straightforward, but well-written, and there are plenty of gags for the kids, but there is also an undercurrent of mature humor to keep the adults from getting bored. As in Rudolph, the characters do sometimes break into song, but if that isn't your thing, they are usually pretty short.
The modeling and animation are great in this movie: the Invisible Man and Dr. Jekyll/Mr. Hyde are particularly well done. Aside from just having the characters themselves, there are other nods to horror classics, so this was obviously written by someone who knew the genre pretty well (aside from the fact that Dracula walked around in daylight, which bothered me a little). It's simple, good-natured fun, and a nice change of pace from the darker zombie films, so if you're looking for something different (and can find a copy), give it a try.