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!
I agree! I’m actually in the process of selling all my old Nintendo games, and each Pokémon game fetches a pretty penny. The only game I haven’t put up for sale is Ruby, simply because of the sentimental value it has. Also it still has my save on it, with hundreds of hours :')