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!
3rd generation, specifically Pokemon Ruby
Hey Pokemon Ruby is my first game too! Definitely my hook into Pokemon, though arguably Emerald has a lot more content, Ruby will always hold a special place in my heart