Star Wars again, because let's face it. That world is so huge. There's lots of planets.
(One could say the same for Star Trek, but they stick in my mind so fleetingly.)
Hoth is a planet that always stayed with me, probably cuz as a kid I was fascinated by the scene where Han has to cut open his tauntaun and stuff Luke inside to keep him from freezing to death. I don't know why, but I was. I used to identify the movies by their opening scenes and always said my favorite was "the one on the ice planet."
Which was Hoth, of course. A planet of ice, snow, and more ice and snow. I could never live there.