It's close between Forza Horizon 4 and Red Dead 2 for me, but I have too give the nod to Forza here.
Beyond that, I'm also getting AC Odyssey and Super Mario Party, but at some later date.
Bet with Intrinsic:
The Switch will outsell 3DS (based on VGchartz numbers), according to me, while Intrinsic thinks the opposite will hold true. One month avatar control for the loser's avatar.