I am tiling my bathroom floor and I need to put in a flase floor before I can tile it what is the best sheet wood to use so I can tile on top of it thank you

Use a cement board, not wood. Hardibacker is a good one to use, comes in 1/4" and 1/2" thickness.

I think you mean a sub floor and I agree with Amricca regarding Hardibacker. Flooring usually uses 1/4" backer with their special 1.25" screws and glue. What is the current floor made from, wood?