Right, The Avengers theme is the most memorable, but I try to recall it myself randomly (like right now) I can’t for the life of me. I’m sure the lack of credits hurts, that’s a good point. Some of the most memorable themes happen during the opening credits where there’s plenty of space to focus on them.
Star Wars is the most obvious example, but I can even remember some of the opening of The Shining, which is just kind of an ominous trombone bit. Hell, I don’t think I’ve seen an episode of the old X-Men cartoon for 15 years but I can recall that one instantly (though TV benefits from the repetition).