Part 1 - you need to watch it, but it's just 2/10
Part 2 - much, much better 6/10
Part 3 - the final boss 7/10
Part 4 - the best villain ever appearing after 50% episodes... worth though 9/10
Part 5 ("airing" for now) - what a villain, what a hero, what a crew. 10/10
I liked JoJo since Part 2, but it was always a little boring for me/not interesting enough. With the part 4 however, It's finally Friday.
The author leanrs from his mistakes. It's very enjoyable to see the same thing done just better.
Part =/= Season, keep in mind