With nostalgia bias, I would say HGSS because I loved the heck out of GSC. But ignoring that... hmm. At the moment, something between that, Platinum and B2W2.
X&Y - well, sure, it wins hands down in the graphics. That said, it's not all that overwhelmingly good in that department. You can notice the framerate issues (who hasn't?), it's not hard to notice that your character and items don't even touch the ground in the overworld if you pay attention (really!), and raving about the usage of 3D models is a bit odd given it took them this long. Meanwhile, Amie and Super Training were all kinds of great, the region was pretty large and... pretty, and it's the standard fun Pokemon game experience (at least for myself).
But it does lack in polish here and there. Gym puzzles are lacking, gone is any sense of decent characterisation for the majority of Gym Leaders and all of the rivals, and while Pokemon plots typically are poor, I felt that this one was more so than usual. =/ Meanwhile, there was a lack of side content for the majority (while I like the Pokeradar, it's not for everyone. Besides Friend Safari (repetitive), the Looker Missions and the battle maison and etc, there's not much side content. No safari zone, no game corner? Not even something like musicals (which I didn't even like much)? It needed more fleshing out.
B2W2 had a worse plot and main rival than BW, but better than X&Y's (not that it's saying much). But what it had over BW was polish. Musicals got an additional movie thing to support it, the entralink missions got revamped and were really fun, we got the PWT, White Hollow/Black Tower were fun and more sensible, the Pokedex had neat upgrades, Join Avenue was an amazing way to promote wifi usage... and a better Pokemon selection as well (not _just_ fifth gen Pokemon until the E4!) There was so much extra stuff to like. And importantly, it wasn't just a rehash of two other games, but an actual sequel. That's something to like and set itself apart from other third-version games.
That is primarily why I like HGSS and Platinum over other games as well - all the games at their basis were fun and gave the standard Pokemon experience. What made those games greater to me was all the extra stuff you could do in them. HGSS had a fancy Pokewalker, added in gym leader rematches, a different safari zone, the Pokeathlon, and improved on a bunch of issues GSC had (e.g. a whole bunch of places to actually explore in Kanto!). And a Battle Frontier. Meanwhile, HGSS had a heck of a good UI system.
Platinum was very polished. DP were good but had a bunch of issues, from a dismal surfing speed to a lack of fire types. Platinum fixed the vast majority of these issues, and improved on the sheer wealth of things it boasted. Contests? Battle Frontier? Berries? Underground complete with a local wifi minigame? 1st gen to have wifi support? The Poketch? Gym Leader rematches? Extended post-game storywise? An actual Game Corner (if you're not European)? The Pokeradar without a 'catch a shiny and the chain resets' aspect? There is so much to do beyond catching and battling. Platinum sure is imo a hard collection of things to beat.
I would say Emerald also gets consideration for being a polished RS (contests! Battle Frontier! Secret Bases!), but it may be better to just wait for ORAS and see what it does with the Hoenn games and considering it as a complete package.
