In the first three films, we see Anakin get stronger, fight better, and at the end of Revenge of the Sith, nearly kill Obi-Wan. Originally he joined the Emperor so that he could gain power from the Dark Side to save his family.
However, after these events, it seems as though the new Vader never got any stronger. We never see him using new or stronger Force powers, never see a bolt of Force lightning, and he even gets damaged by Luke (who was barely trained and should have never come close). Sure, he blocks a blaster bolt with his hand, but that wouldn't be enough to convince anyone he was "stronger", not to mention the rematch with Luke put an end to Vader.
Was Anakin/Vader at his strongest during the final Obi-Wan fight in Revenge of the Sith? Did he ever grow in power after that?