RotS Vader vs OT Vader
Ok, I know that most of you will think that I am insane making this a battle, but hear me out.
OT Vader has far more experience than RotS Vader does, he is calmer and thinks things out more.
Yes his blows are slower, but they are more powerful.
He also has far more control over the force than RotS Vader does. He is able to duel Luke in ESB and throw dozens of the poles at him. Impressive since most people can only do one or the other, he had to focus on attacking and controling a dozen objects. OT Vader can also choke a person from 8 miles away (choking Ozzel on the Executor), RotS Vader had trouble choking a person who was directly in front of him. It may be because that person was Padme, but at that point he was not thinking and was completely consumed by the dark side, I do not think that it would matter much.
Also, OT Vader has been training under Palpentine for 20 years, he knows far more of the dark side and of the force in general than RotS Vader does.
Finally, it never says that the lava made Anakin weaker, just prevented him from growing more powerful. However OT Vader has gained control over those powers.