I think Michael Gambon was really good in Prisoner of Azkaban but only in the odd scene since then. His best overall take is in Half-Blood Prince though I still don't like how they dealt with that story. I'm sure his scenes in Deathly Hallows will be great too, something extra special.
I thought Richard Harris was too cheesy as Dumbledore in the first two, but that wasn't really his fault. It was all very kiddified then and they gave him too stereotypical a look and a way of speaking. Alfonso Cuaron then came in and delivered exactly what we needed in the way everything looked and felt, Dumbledore included. Cuaron remains one of the most important people ever involved in this franchise.