Funny this thread asks this question, as i have been thinking about this in recent days. for me, ultimate took my vote. I've loved smash to death since n64, but even though ultimate gives me every character, tons of stages and an amazing soundtrack, i feel the game is taking it's self to seriously with trying to be a competitive fighter with all the attention to frame data, techs and combo ability, and lesser on the ridiculous fun factor that we fell in love with during the n64 through brawl era.
There's nothing wrong with it going this direction, but as a fighting game fan, I want my streetfighter, tekken, dbz to feel different from smash. I also really miss things like break the targets, board the platforms, home run contest, smash run etc. even all-star mode got gimped.
Eh... as a mostly competitive player, I have to say the casual "ridiculous fun factor" is by far (very far) the best it's ever been in Ultimate. I've actually had loads of fun playing free-for-alls on wack stages and even with items on, something I never ever turned on on purpose in Smash 4. I don't have a lot of experience with 64/Melee/Brawl, but in the little I did, casual play wasn't any better, either. Ultimate is by very far the craziest and most fun Smash game, with a great selection of items, awesome Pokémon and assist trophies, the freaking stage morph (seriously it's so cool), and 8-player Smash you can actually play on any stage this time.
Furthermore, it's also by far the best competitive Smash game, balanced to the point where over 90% of the cast is arguably viable at mid-high levels, and even at top level you still have plenty of variety. There's deep mechanics and there's plenty of interesting tech to be found with it, as well as a general focus on more aggressive play rather than defensive. And no, it still feels very much like Smash and not like other fighting games as you mention.
As for the other modes, it seems really silly to complain about missing modes when they've already been absent for a decade now, but apparently there's a lot of people who feel that way. Smash Run is insanely overrated, honestly, though I do recognize there was potential there for them to make a really cool mode. Home-Run Contest is quite a classic and I do miss it, but let's be real, it just doesn't matter. Break the Targets and Board the Platforms have been gone for ages and they're not coming back, because they're just not gonna do individual stages for 80 characters. The one I truly miss and hope returns in some update, though, is Stage Builder - me and my friends had a particular custom stage we loved playing on and it would've been amazing in Ultimate, especially with 8-player Smash being available everywhere.