Obviously the next one is Smash. Seems bizarre people are mentioning mario party. I'm sure that it will sell a few million but its not gonna still be selling years from now, heck there will I'm sure be at least one more mario party on Switch later on to replace it. Pokemon Let's Go will sell very well but as a side game it won't be evergreen and its sales will likely plummet within a year since the real Pokemon is coming next year.
So just Smash for 2018.
2019 will be Animal Crossing if it hits that year, obviously Pokemon, and I would have said 2D Mario but a port will not suffice.