Mine is Pokemon HGSS (the amount of content is mindboggling), Pokemon Platinum (grandest Pokemon experience) and Pokemon Black 2/White 2 (most fun and unique playthrough). I’m also looking forward to buying Legends: Arceus on the Switch, have heard many great things about it!
My all time favorite is Pokémon Emerald, but I also really like the ORAS remakes for the newer pokémon, updated move sets, and EV training. Legends: Arceus is one of the few modern games I really liked. Finishing the Pokédex in that game is really fun.
omg definitely the ORAS remakes!! How could I forget them, best plotlines arguably! I just love how villains in those games have their own redemption arcs