Lucas says OT Vader is a shell of what he used to be. he said in ROTS we see Vader in his prime.
These are vague statements. "Prime" does not have to be about battle, it could be about glory, influence, etc.
He is a shell of his former self! He's a cyborg! He's a man in a shell!
It says in Shadows of the Emperor than Vader wished that someday he would be the man he once was, physically.
In that exact same book, he claims to be more powerful than he ever was as a Jedi. However, had he stayed human for the 25 yeats, God knows how powerful he would have become.
Lucas says after donning the suit that Vader is never again as strong as the Emperor.
Neither was ROTS Anakin.
ROTS shows Anakin with suberb speed and excellent saber skills, killing Count Dooku while the OT shows Vader getting all emotional and losing to Luke
Good speed, but speed doesn't decide everything. Mace was slow, but powerful and strong. And he managed to take down Sidious. Killing Count Dooku might have been result of Dooku arrogance and underestimation of the boy.
Well, can't blame him for loving his son!