RE: Grey Jedi

Who was the master or first person to start the grey jedi order/clan etc ?
Also who is considered the most powerful grey jedi/master in cannon and legends ?

No idea about your first answer. The most powerful so far is likely Ahsoka right ?

Depends what you mean by "grey" Jedi, because that's not a category of Force users in canon.

If you're talking about Force users who use the dark and light sides of the Force, then the Prime Jedi was likely the first.

Then you have others like Ahsoka and Ezra(even though they're still primarily aligned with the light side).