This is a no brainer. Either Samsung Galaxy Note 8/9 or Samsung Galaxy S8 Plus. I don’t think there’s anything better on the market atm.
There are two things to look at when searching for phone for VR – resolution and power. Minimum you should look for is Snapdragon 835 or equal processor with at least 4GB RAM. Resolution has simple rule – the bigger the better. And there’s a compatibility thing. If you buy Samsung you’ll be able to use Gear VR, other phones are limited to Google Daydream and Cardboard.