Now I'd say ROTJ was Vader's prime:
"Lord Vader did not mind waiting, though, nor was even aware of it. For it was an honor, and a noble activity, to kneel at his ruler's feet. He kept his eyes inward, seeking reflection in his own bottomless core. His power was great, now, greater than it had ever been. It shimmered from within, and resonated with the waves of darkness that flowed from the Emperor. He felt engorged with this power, it surged like black fire, demon electrons looking for ground...but he would wait. For his Emperor was not ready; and his son was not ready, and the time was not yet. So he waited."
-- Return Of The Jedi
"Even his own generals could not escape Vader's wrath, and as time went by, the Sith's powers grew even stronger."
Luke definitely made Vader more conflicted which affected his duel with him, but overall his power was at it's peak as of ROTJ