Just my two cents. The Doctor was the best-acted male character in all of Star Trek next to only Jean-Luc Picard.
Picardo single-handedly took a "blank slate," backdrop kind of character and ran with it. Amazing accomplishment on his part, and it's awesome to watch the Doctor develop as the series goes on. He's better than Data and I believe they gave The Doctor a personality/emotions and not Data because Picardo had the acting chops to portray it.
Bar none, "Joe" grows more than any other Star Trek character over the life of a ST series.
Once Seven came along the two of them (Doc and 7) for all intents and purposes carried Voyager on their backs. Not that it would have failed without them but they kept it a "big league," "dinner table" sci-fi show because of their big-time acting talent.