Stage design is the one thing that has consistently gotten worse and worse from one Smash to the next.
-Smash 64 has great stages. They are very spacious and they rely on interesting platform layouts and on visibile, but overall unobtrusive gimmicks.
-Melee is also great, offering some of the series' most memorable stages (Onett, Temple, Fountain of Dreams, Pokèmon Stadium, Jungle Falls, Mute City). The more "involved" stages still have simple and understandable gimmicks (like Brinstar Depths or Big Blue). It's still focused on interesting layouts and manageable gimmicks.
-Brawl is where something breaks. It becomes way more hit or miss. We still get some good old-style stages, like Delfino Plaza, Frigate Orpheon, Smashville, Halberd... but it's also where the gimmicks sometimes become too overbearing (just compare Pokèmon Stadium 2's transformations to Pokèmon Stadium 1), where the platform layouts start to feel randomly put together (compare New Pork City to Temple), where citationism starts to override fun (looking at you, 75m), where some stages start to feel cramped (Yoshi's Island but also Luigi's Mnasion), or the basic ideas just aren't that fun (like Spear Pillar or Green Hill Zone... why does the ground have to break?). Though overall it's more good than bad.
-Smash 3DS stages are actually pretty good, pretty Melee-styled.
-Smash Wii U stages however are horrible, the nadir of the series imho. Stages are either borderline unplayable (Pyrosphere, Wily Castle, Orbital Gate Assault, Great Cave Offensive) or incredibly boring and forgettable (I mean, Coliseum and Wii Fit Studio are basically the same stage - flat walkoff with random platform layouts appearing sometimes), or just too clunky and weirdly put together (Suzaku Castle). Also, you get transforming and travelling stages like no tomorrow. Pilotwings and Migar are the best things to have come out of that game, imho.
-Ultimate has few new stages, and though they dialed it back on the craziness they kinda feel basic and barren. On top of that, Moray Towers and Great Plateau are borderline unplayable. Spiral Mountain will probably be the best new stage.