I like It Happened One Night. It's got a lot more energy than Roman Holiday, an a much less baffling plot than The Philadelphia Story. It from a time period that you don't see a lot of live-action movies from, and so has a very cartoon-y feel. Claudette Colbert's hair looks Betty Boop-y and everything.
It's interesting to me that this movie has a very similar plot to Roman Holiday - a reporter hangs out with a run-away aristocrat, plans to sell her story, and then falls in love with her. Peter and Ellie are much more charismatic than I-forgot-their-names, though, and the movie doesn't take itself so seriously. It Happened One Night knows that it's sort of a silly movie, probably because it's just a cheap blockbuster, and that makes it much more charming. It allows itself to digress into discussions on donuts and hitchhiking without pretending they mean anymore than amiability between the leads. This movie doesn't treat every little scene as if it were weighted with the romance of the century, and that I think is what makes it so good. I'm of the opinion that a really good romance needs some degree of naturality, to make it both believable and something you can root for. A quality romance is one where the people seem to be actually happy when they're around each other.
I'm surprised by how much younger Clark Gable looks in this movie - it's only five years before Gone With the Wind. It only just struck me that he should be kind of smarmy looking, what with his mustache and his self-righteous chracter roles, but somehow he isn't. That's some charisma for you.