Seasons 1 and 2 I would give a solid 5. Lots of good dialogue and character development. It didn't feel rushed and I thoroughly enjoyed it.
Somewhere in season 3 I started feeling annoyed of the show. I'd give that season 2 stars. It was watchable but I was starting to get annoyed with Uhtred's bad decisions and how easily he would escape death. It felt like sloppy rushed writing. They started introducing new characters that I just didn't like and not their characters but the actors. The events were beginning to speed up while the story line slowed down a bit.
season 4 I give 1 star. The addition of new characters and the pace of the major events had my head spinning and I was even further annoyed by Uhtred's love interests and ability to worm out of trouble while placing himself stubbornly in harms way again and again.
Season 5 I would give 5 stars. I didn't like the direction of Brida's character in season 4 but her acting skills are pretty awesome. When she cries I feel it in my heart. I wish the show had been longer and maybe season 4 would have been better. It was just too much too fast so it was hard to care for the characters especially new characters they didn't really build up. But season 5 and what they did with Brida won me back.