David Gilmour of Pink Floyd. He never sucked, imo, but as the years grew he just got better... He's in his 60's now, and he still sounds how he did in his 20's, if not better.
Well, Bruce Dickinson from Iron Maiden sure got a lot better with time. He was so lackluster in his early years and didn't have a proper breathing technique, he also had troubles reaching high notes. He is A LOT better today.