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!
hgss was my first proper pokemon experience and still holds up to this day, but i absolutely love the storyline and self-contained nature of black and white. audino is one of my absolute favourite pokemon and i remember teaching it a ton of OP moves (Hyperbeam, blizzard, etc) on my first playthrough