Tilers: What's your favourite surface for tiling onto in a bathroom?

[albaPhenom]

I'm going to board out my bathroom soon, floor and wall... what surface do you most prefer tiling to?

Name check products by all means.

I want to make life as easy for the tiler as possible and have the backing board be belts n braces from a waterproofing standpoint.

 

[raykf]

This product worked well for me:
https://www.insulationexpress.co.uk/marmox-multi-board