BW2, and it's not even close. Look, I love HGSS and all, but it doesn't serve the role of a sequel very well. If you look at what it did to Kanto, it's actually kind of a step backwards, even with HGSS' enhancements over GSC. You lost a lot of areas from the originals even in HGSS (Rt. 7-8 Underground Path, Power Plant, Pokemon Tower, Rocket Hideout, most of Silph Co., most of Cinnibar Island, Rt. 23) and they didn't do much to actually improve Kanto (they mainly just added the Trainer House, and the Radio Tower and House of Memories which are basically just one room buildings gameplay wise). Pretty much all they did in terms of enhancements was tack on Johto which was an equally shallow region that hung off Kanto like a conjoined twin. Conversely, BW2 took Unova and drastically improved it. They added a ton of new areas, a ton of new Pokemon, and a ton of new extra features, and Unova felt miles better as a result. By focusing on the one region they were able to get much more out of it and it felt much more polished as a result, and I'd strongly prefer that model. I'd rather have one polished and well designed region than two mediocre ones.
I would definitely love to see more BW2 sequels, and I feel like after they made BW2, they almost felt like a necessity. They opened Pandora's Box with BW2, it's so drastically better than traditional third versions and remakes that there was no going back to them without it feeling like a step backwards for the series (and the likes of ORAS, USUM, and LGPE really did). About the only other thing they could do to enhance regions that didn't was DLC. But in terms of going back to a previously established region with a new game, BW2 sequels should've been the new normal, and it's almost criminal that they've been one and done because there's so many other regions that could use a similar massive facelift (Kanto, Johto, and Kalos most notably). They don't even need to really be sequels in the sense that they take place a set time after the original adventure if that complicates things (although they could get around that for older regions by bundling a digital version of the original with the sequel), just being a massive reimagining of the original region would help a lot.