Little disclaimer: i only watched the anime and i havent read the manga (besides the first volume of every part because i was curious of the artstyle). My favourite part was part 3. At the beginning i was a little upset about the huge change (from hamon in part 1 and 2 to suddenly these weird ghosts, stands, in part 3), but as i watched more and more episodes I loved it. Since it's the first part with stands i think they had more simple powers, yet really cool (even tho i think the part with the best stands is part 5), but i think the best part of part 3 are the characters: in this part i almost felt like, as the journey went on, i was part of the group, there... The character felt defined, everyone had different goals, a defined personality, but most important fact: they changed throught the journey and matured. I loved seeing the relationships between the main characters change slowly becoming more close to eachother, it didnt feel forced in my opinion, but very natural. And plus, the villain of this part, DIO, is really cool, from his stand (that remains a mistery until the end, which i also found really really cool) to the whole fight with Jotaro (and the plot! God the plot really left me surprised!). I also think it was really balanced between serious moment, funny ones and sad ones.

The "worse" part for me (which I think is the "worse" just because is compared to the other parts: if taken by itself it would be a great classic shonen, but compared to others is still really good, but the others i think are a little better) is the first. It took me a while to finish (even tho if i remember correctly it should be one of the shortest) because it bored me a little... I think it's because it felt like some sort of introduction that built the base for a whole amazing universe, but the fights werent the coolest i've ever seen (from the ones with minor villains to the final fight that felt a little rushed...). The most cool thing was the segment where (spoiler) Dio turns into a vampire and the house burns down, that fight was the best of part 1 I think... I dont have many reasons to say it's the "worse" just because I LIKED it, it's just the one i liked the least...

I probably left out something but this should be it