For me Platinum is the best enhanced version, mainly because I only played USUM and Emerald as well and I never beat either of them (I do like them both though, genuinely). The beauty of this is that what makes these games our favourites is what we felt was worth adding to already solid games - I'm not a massive side-quest fan or online player in Pokemon (so to speak) so things like the photography stuff in USUM and the Festival Plaza felt annoying and forced to me, whilst I do enjoy stuff like the Battle Frontier and move tutors so the games with easy-enough access to those were good for me. Funky times.
Anyway, Platinum is awesome because I like a decent challenge in my games alongside some actual options for a team and Platinum was an upgrade in both of these areas from their sister games (even if it took away Honchkrow which I am very bitter about). Flint/Volkner/Candice using not enough team diversity? Slap in the new evolutions for past-gen Pokemon. Like seriously why weren't they added prior, why lock them to the post-game? Stuff like Electivire, Gliscor, Tangrowth, Yanmega, Porygon-Z and Gallade are awesome to use and we had to wait until Platinum to get them? Gross. But Platinum fixed this and added them to the teams of important characters. Said gym leaders became more challenging with their improved teams and I really liked that their gyms were updated too. Surf speed was increased! Move tutors were available via shards which were reasonably easy to get (with no digging I could get around 2 tutor moves on my team which felt pretty solid). The post-game gave us gym leader rematches and a fully functional Battle Frontier! It was awesome.