It's hard for me to rate this show based on stars. I would reserve 5 stars for my few absolute favorite and mind blowing shows and movies. This is close to that for me. I would rate 4.5/5 if I could.
This show got me emotional and is visually stunning. Like other people have mentioned I did feel it fell off a bit around season 5. At first I was disappointed it was transitioning to focus heavily on the next generation (Ragnar's sons). I missed the old characters of course.
However, after feeling it was falling a bit flat it picked right up again. I ended up appreciating the acting of Ivar's character where before I was critical of him. I was hooked again pretty quickly.
The only critique I have is that I felt they overly sexualized the show a bit. I feel weird saying that as I'm not offended by sex at all, I just felt it was a little over the top. At times the characters themselves came off too clean almost. The viking shows I've watched since (Valhalla, Last Kingdom) didn't sensationalize in this way and I appreciate them more for that.
This was a stepping stone for me to get into other viking shows and movies and I love it for that.