I was going to try to limit myself to one Universal Monster movie for this list, but when I was looking at '40s horror movies, Universal dominated my favorites the same way they did with the '30s. Just for a different reason.
Universal monster movies in the 1930s were still trying to be genuinely scary and that carried over a bit into the '40s. But by the middle of the decade, they were getting pretty goofy. That's not necessarily a bad thing though. I like both the serious and the silly versions of these characters.
The epitome of the silly side of Universal horror is Abbott and Costello Meet Frankenstein. It's such a funny film, but not at the monsters' expense. It keeps me hooked by treating Dracula, Frankenstein's Monster, and the Wolf Man as actual, scary beings and not objects of parody. It even continues Larry Talbot's tragic quest to end his curse in a way that jives with everything that's come before. The humor is all in Abbott and Costello's (especially Costello's) reactions to them, as well as just some very funny jokes ("Hey, you're gonna have to get your dog away from the phone. I can't hear a word you're saying.").
And of course, though the three most popular Universal monsters had already appeared together in House of Frankenstein and House of Dracula, this was the only time that Bela Lugosi got to join in the team-up, finally reprising the role that made him famous. The only way it could have been better would have been if Boris Karloff had played Frankenstein's Monster, but I also like Glenn Strange (who'd been in the two House of movies) quite a bit. No complaints there.