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, IMO the franchise kinda started going downhill after that.
And in the state it’s in now, I don’t think I see myself buying a mainline Pokemon game ever again, sadly