He's weak
The PT has affected my assessment of Vader as well. Not so much from the standpoint of how powerful he was, but from an emotional, mental and psychological perspective.
In the end, I see him as weak. Not weak as a Jedi or Sith, but weakminded, easily manipulated, wrought with fear, foolish; and later, filled with anger and bitterness.
Some might argue that it's natural to find these characteristics in an evil man like Vader. On the one hand, I agree. But in the OT you see only power and a quest for more power, driven ostensibly by ambition and the ruthless perspective that powerful men are entitled to take whatever they want.
In essence, the Vader in the OT is a powerful man on a quest for more power for power's sake. But from the perspective of the prequels, you realize that Vader is a failure in his own eyes. It 's true that Vader remains one of the most powerful Jedi/Sith in an absolute sense, but he never planned to sit second chair. You realize looking back that Vader was on a ruthless quest for power because that was all he had left.
Vader's backstory changes our understanding of the nature of his quest. There's a difference between seeking power because you want more and seeking power because you can't have anything else.
Anakin sacrificed everything to save Padme and rule the galaxy. He knew that with additional study and mastery of the force, his powers would far surpass Palpatine's. It would only be a matter of time until he sat on the throne.
As he lay helpless on the banks of the volcano, pitifully sliding into the lava, he knew it had all been for nothing. Palpatine admitted he couldn't save Padme, Vader himself was responsible for her death, and in the wake of his own egotism and poor judgment he knew he could never fulfill his ambitions.
In the end, he was left a bitter, angry man tormented by the reality that he would never be the man he should have been.