I grew up playing Gens 1-3 and played every game since, and I personally thought Legends: Arceus was the most fun I've had playing a Pokemon game in years. It's not a "traditional" Pokemon game in that there's no gyms and not many trainer battles, but in my opinion it was the best thing Gamefreak have done in at least the past decade. I highly recommend it. It's in my top 5 Pokemon games of all time.