On the windows store, sea of thieves is already the highest selling Microsoft published game ever. Already ahead of forza horizon 3, gears 4, etc.
edit: also, for anyone wondering, "fastest selling" doesn't take gamepass into account,
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.