Overall, Emerald is the most fun for me. Hoenn is an easily replayable region and feels fun, free, and lighthearted - no pretense, no subpar dungeons, good level curve. The story is good without being heavy handed, the regional Dex is one of the very best, and all the environments are fun to explore. One of my favorite things to do in Hoenn is find the most remote corner of the routes around Fortree or hidden up on the north side of Route 115. Most importantly, the Emerald Battle Frontier offers hundreds of hours of legitimate, replayable challenge - I still consider it the gold standard for a Pokémon postgame experience, honestly.
I think the runners up for me are Crystal, for the pure nostalgia factor, Johto in general, and the first Battle Tower; Legends, for the unique mechanics, the excellent story, the region and lore, and the visual style that I actually like a lot; and Moon, for the story and characters, both of which are better there than in any other game in the entire series. If I’m looking for those particular experiences, I pick those up.