Best Garry's mod Addon ?

Doesn't that kind of depend on your personal taste? Not like there is a general best, if that is what you're looking for just use one of those mod websites and sort by best... the addon on the top will be the "best" one in that case. Considering it is liked by "most" people or has "most" usage. Though I can't guarantee you'll that that is what you're looking for.