Hard to say. Old ones are more challenging, but have aged visually poorly and lack many QOL features that the newer ones have. They feel like a chore to play because of that. Newer ones arent pretty to look at either, and they are more aimed at a very young audience, but mostly - are horribly easy. Theres zero challenge to them. My personal experience is that I lost interest very quickly because of that, there is just no fun in an RPG game that doesnt challenge you at all.
The last one I enjoyed quite a lot was XY, but mostly because it was very oriented to the meta multiplayer community, without that its probably not worth it at this point. Legends Arceus was fun, so I guess that'd be my pick? Or maybe wait for the Legends Z-A to drop, see if it has good reviews







