It has been a while since I've watched the older anime, though currently rewatching Kanto. I feel like Johto added a lot of meaningful locations. While Kanto did add a lot, I feel like they were just thrown in for filler. Similar to Decolore Isles. And I don't want to count the Orange Isles because it is its own region similar to Decolore.
Hoenn and Unova didn't add many places, nor were they largely memorable Sinnoh was similar, but it had more impactful moments. Alola just didn't add much nor were they very meaningful.
Kalos is second. While it added few area, almost all of them had an impact on the characters.
Honestly? Kanto. I love how memorable some of the locations are in the anime when Kanto is concerned. Vermillion is a massive city with a big presence in Journeys and Cinnabar Island has always been memorable for me because of the inn and cool volcano in the background. Cerulean City has a nice view with the river too. The region is largely forgettable in the games design-wise, but the anime does a better job for sure.