I know it sounds very cliché, but the movie of the joker catapulted Phoenix's career even more, as it was like a new achievement for him. Of course I have to admit that his other roles were also very prominent, like the gladiator, but he was under the radar.
In gladiator his role was very good, as you say he showed a person totally corrupted by power, and that was great, and in guason he is a person who doesn't care about anything, something totally different from cesar, no doubt this actor knows very well how to make legendary roles.