Pokemon Legends: Arceus if you like to play story mode. (as Bigman17 said, once the story is over its.. over.. kinda)
Pokemon Sword/Shield if you want to do any competitive PvP, since its the current PvP Hub Games iirc.
Pokemon Lets Go Pikachu/Eevee if you want nostalgia (I liked the shiny hunting there a lot, too.)
Any Pokemon Game is a great pick up, imo!
Already pre-ordered the new games