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!
Ruby/Sapphire, ORAS as a close second, then HG/SS.
Gen 3 was my absolute favorite and I was beyond hyped when I learned it got a remake