Perfect in Deathly Hallows!
Part 2 showed he finally understood Dumbledore and gave a brilliant performance despite being in only a handful of scenes. I also thought he was terrific and captured all of the right qualities of Dumbledore in Half Blood Prince.
He was decent in Azkaban, outright bad in Goblet of Fire(thanks to the bad director), was more mellow but still not perfect in Order. But by Half Blood Prince and Hallows part 2, I was convinced that Sir Michael was the right man for the role.
He also seemed to be wearing dirty rags in Year 4 but the costumes got much better by Order of the Phoenix. He even seemed to be channeling Gandalf the White in Deathly Hallows, costume and all!