It's entirely possible that nostalgia has a lot to do with it, but I always liked the stage layouts in the original Smash Bros as much if not moreso than anything else the series has done since. The original Peach's Castle with those angled walls, Hyrule Castle was long but had some nice changes in geometry...A lot of them obviously feel more simplistic in approach than more recent stages, but I liked it that way. There are too many now with needless background transitions and crazy crap happening, and while those can be fun nods to the games they're inspired by I think it kind of hurts the focus that should be on the actual fighting.