Season 4
It's the general consensus among fans that season 4 is the best. I disagree. I found it lacking and at times downright boring. I'm not sure if it is because they crammed the events of one of the longest books into only 8 episodes or because the heart of the show is the interaction between Ross and Demelza that couldn't happen as much in this season with Ross being in London half a year at a time.
It could also be my own fault because this was the first season in which I viewed it after I read the book it's based upon, The Angry Tide. The book was riveting. I couldn't stop tearing through it at breakneck pace. It is a masterpiece of plot. However, the season didn't measure up, but the screen adaptations rarely do.
Here's my ranking of the seasons:
1. Season 1
2. Season 2
3. Season 4
4. Season 3
It will be interesting to see how season 5 shakes out. This season is based on the writer's imaginings of what happened in the ten year gap between books 7 and 8 of the series. Maybe it will have that magic again like the first two seasons did for me.