I think people overrate the fukk out of GOTG. Decent movie, but wow, I don't understand the insane love it gets. Wouldn't even place in my top 20.
Batman Returns is way too high. Watchmen was boring. I'm sure they could've come up with something else and left Thor and Hulk off the list altogether. Hulk was another bore-fest and Thor is the epitome of a mediocre superhero movie.
Superman 1 & 2 definitely deserve a spot in the Top 15... they are iconic but they aren't great, if that makes sense. I re-watched them both recently and they feel more like made-for-TV movies, and I'm not referencing the special effects.