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!
Favorite one was probably platinum.
Favorite handheld spin-off was PMD explorers of sky.
Favorite console spin-off was Pokémon Battle Revolution
I’m so glad you mentioned Pokemon spin-offs! Have always been interested in those but never got time for them :(