I really liked season 6. It's a contender for my favorite season.
Season 7 starts off really strong but peters off halfway through. The last episode was pretty good but not great.
One day I'll have to go through the entire thing again and reassess it. I remember thinking that Season 3 was great at the time but in retrospect it had a lot of weak episodes. My wife recently went through most of the series (up to S6) and did not like 3 at all. "The Doctor Is Sin" is probably still one of the strongest single episodes though