Well, if you ask Davros, Daleks are the superior race, so all the Daleks trying to conquer the universe are already good. :)
But, even given a more reasonable definition of good, there are plenty of examples in the show, from Alpha, Beta, and Omega in "Evil of the Daleks" to Oswin Oswald in "Asylum of the Daleks". The former were turned good by being given the "Human Factor". Oswin turned good just by resisting her Dalek conditioning even after being converted to a Dalek. Other ways Daleks have been turned good include a polarity-reversed "Dalek Factor", getting some of the Doctor's DNA in them, or going mad breaking through a time lock and then realizing that the Daleks' proper destiny is self-destruction.