Only selecting one film this time, because while there are lots of films that make me happy, there's really only one film that I'd answer for this question instantly, where the others I'd have to think about. That would be The Dirty Dozen (1967).
This is a film that I can watch at any time of day, any day of the week, heck, every day of the week. If I'm in a less than stellar mood, it'll have me smiling again in just a few seconds. It's impossible for me to watch this film and stay in a bad mood. Just thinking about it makes me happy.
Now, you may be thinking, is she kidding? It's a war movie, nearly everyone dies including my favorite character, for goodness sake, how can this film make her happy? Well, a big part is that it's tied up with my sister and family, and many, many happy times spent watching it together over the years. And the rest of it is because it's simply a really good movie with non-stop great, quotable dialogue, acted by a whole lot of actors I love: Lee Marvin, Ralph Meeker, John Cassavetes, Charles Bronson, Clint Walker, Ernest Borgnine, Jim Brown, Robert Ryan, George Kennedy, etc. And it's funny. The exchanges between characters just crack us up. Honestly, I don't think a single day goes by that a line or two of dialogue from this film isn't quoted between my sister and me. It's that much a part of our daily lives. Even normal stuff, like "Come on," or "Yeah," we usually say with the character's inflections, just to make each other grin.
I don't think there's any other film ever made, not even among my favorites, that puts me in the same Happy Realm ballpark as The Dirty Dozen.