So admittedly I've only played 3/5 of the options (and only beaten 1 of them) so I can't really comment on whether Yellow or Crystal were any good. I never beat USUM but have played Platinum a lot and I gave Emerald a good go a few times! So for me my vote goes to USUM, just based on what I've played. I will say that if Yellow is anything like Let's Go (I know Let's Go was inspired from Yellow although the characters are slightly different) then perhaps that clinches it but I will leave that be for now.
I don't mind USUM, I think it adds a lot of cool bits to the games that the originals sorely needed (move tutors, Mantine Surf, fleshing out Necrozma a bit, being a genuine challenge from the beginning) and although I've never beaten it I think I would prefer to play it over the originals for the extras I mentioned before. However, it also does a fair bit which is pretty damn annoying! It does not deal with the fact that the game is full of cutscenes and tutorials which annoys the hell out of me, particularly during the Melemele stuff when I really do not care much about how to play or the fact that it really wants me to try the Festival Plaza or the photography stuff (or the posing you do before Akala Island).
It changes the Pokedex up which is both good and bad - new Pokemon like Inkay, Zorua, Buneary, Hawlucha, Noibat and Murkrow are now available much earlier at the loss of stuff like Misdreavous being removed to the late/post-game, alongside stuff like Poipole being a late-game option despite being a brand new Pokemon you encounter as early as Akala. It also apparently changes the story to better justify Lusamine, which from what I've seen/looked up kind-of sucks since she was a cool villain in the Sun/Moon games and now it's just kind-of... eh.
The biggest thing is that it wasn't worth the full price of a new game for very little change - the new additions and minor story changes should've been a DLC really (and probably would have if these games were on the Switch). Would've made for a cool one too in the post-game, if you took Necrozma to the altars or maybe you would have encountered the Ultra Recon Squad hunting for Necrozma in the post-game and been battled with a Poipole/Naganadel, perhaps a friendly one would've followed you around and guided you in the Ultra Wormholes and you've caught the one you befriended in the end. Maybe if you had Nebby it would've changed things up a bit, who knows? But yeah, USUM is my pick for a bad enhanced version. Platinum was wayyyyyy more awesome.
