7. Riddler Reformed
Of course the biggest change to happen - and then be totally undone - in the storied life of Edward Nigma is when he abandoned his superciminal activities altogether, and even ended up helping Batman for a while. It didn't work out in the end, as it hadn't for Two-Face, but you've got to give the guy credit for trying, at least. It's more than most of the Gotham rogues gallery ever manages.
In a story written by Paul Dini, one of the masterminds behind the nineties animated series, the Riddler Reformed story arc came not long after the culmination of Hush and is violent aftermath. After having been in a coma for nigh on a year, the former villain awakens to find himself cured of the insanity that saw him compulsively committing crimes. Whilst he does lose his madness, he also manages to hold onto his immense intellect - specifically, his genius-level capacity for solving puzzles. Which makes him not only a pretty good asset in the fight against crime but not a bad partner for Batman, the self-proclaimed World's Greatest Detective (we assume it's self-proclaimed, anyway, because we don't think we've actually seen anyone call him that).
It probably helped that during the time he spent unconscious he totally forgot that he knew Bruce Wayne's secret identity, which was pretty handy. At first Riddler acted as a private consultant, hired to figure out crimes and mysteries that nobody else could fathom. Inevitably this leads to him having encounters with both Batman and the extended Bat-Family, slowly earning their trust and eventually becoming a valued ally in their fight against the scum and villainy of Gotham City.
It was a neat twist on an underused character, and made a lot of sense...right up until the point somebody decided they were bored of the reformed Riddler and sent him back into being a bad guy for no real reason, and without much explanation.