I think Melee's stage design still feels so great because it encourages you to use level geometry strategically in a way that later entries haven't replicated often. Temple's cave, the solitary rock in DK Jungle, Onett's ground level as opposed to the rooftops, the various sections of the Great Fox, the central column in Peach's Castle (etc.) are all deliberately placed there to impose a certain flow to the battle. You can retire in a section of the stage that has more walls when you're about to die, and viceversa bring your opponents in the sections that leave you more open when you're trying to score a KO. Stage hazards were something to keep in mind, but they rarely dictated the flow of the Battle, they were more something to pay extra attention to.
Brawl shifted its philosophy to very basic stage layouts where stage hazards are way more intrusive, and from there it's been downhill.
Now Brawl itself has generally good level design though, and quite a few bangers: Wario Ware, Aero Dive, Castle Siege, Halberd, Delfino Plaza, Pirate Ship...
In Smash 4 they dropped the ball. Stage bosses are just the opposite of fun, some stages barely feel designed at all (Coliseum and Wii Fit Studio are basically a reskin of each other), and what's up with the big stretches of very tiny platforms placed in absolutely random positions in Gaur Plain and Palutena's Temple? Pac-Land is a mess, the hazards in Kalos League are waaay too obtrusive...
Those stages are not used by competitive players, but in my experience they also scare away casual ones because often you die without even understanding what killed you.
Though Magicant, Midgar and Boxing Ring are really good.
Ultimate's stages are kinda there, you don't have the strategy of Melee nor the chaos of Brawl... But they're still Better than Smash 4's.
I like in particular Dracula's Castle which is basically a Melee stage, and KOF Stadium and Spiral Mountain for their simple, natural gimmicks (+ nostalgia in the latter's case).
And yeah, Northern Cave. Plays fine and is a spectacle.
To put it short... Yes, Melee is where Smash stage design in general peaked imo.