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!
Ultra Sun/Ultra Moon, because it has the most available Pokemon (no Dexit). It has Mega Evolutions and Z-Moves. It has a fun Battle Tree and good breeding mechanics. For what I like to do in Pokemon post-game (breeding competitive for battle facilities), it offers the best selection.
For running through the game itself, probably HGSS (big fan of gen 2 since I started with Silver) or Black/White.