What is the best monk build?

Define "best". What do you want to do with your monk? Which parts of the class's schtick do you want to emphasize?

The best monk build is the one you have fun with. 

Are you playing 2014 or 2024?

The best monk build is the one that jumps out of your mind, put some scores on a character sheet, grab some dice and have fun. That’s how easy to have the best build.

The best build is always the one you have the most fun with. 

If you mean highest damage potential in a single class build then i would rank the 2024 phb monks

1) elements monk 

2) mercy monk

3) open hand 

4) shadow monk 

Oo