It will be close, but I think PS4 will take it. PS4 is due for a price drop which will help it a lot this holiday. Switch will have Smash Bros and Pokemon Let's Go, which will help, but I don't think Pokemon Let's Go will shoot to the top of the sales charts like a mainline Pokemon game would. But Pokemon Let's Go has the potential to have really long legs. If the Pokemon Go fans like it, then it could sell for a very long time, but I don't think the core Pokemon fans are going to rush out and buy it immediately.
Anyway, I'll give it to PS4 this year, but it will be close.