The phrase "top league" means being at the highest level or best group in a specific field or activity. It refers to the highest rank or most prestigious category. Full definition
Judging from his posts and interviews, the old timer is pretty confident that he will hold up to the current top league of bodybuilders. (fitnessandpower.com)
He's aging and moving to one of the 2 top leagues for the first time. (justarsenal.com)
Why the obsession about topping the league table or not? (justarsenal.com)