Part 1 and part 2 are very outdated, it's the sin of the late 80's manga indusry, not Araki's fault. Part 1's ending originally also leaves Dio's fate up to the reader as he could be dead or not, it's not that bad if you don't know about part 3.
Part 3's ending was an asspull, that's a fact.
Part 4's ending was kino, that's a fact
Part 5's whole point was the requiem arrow and the fact that piercing your stand with it grants you what you desire most. Giorno wanted to defeat Diavolo and become the boss, and the arrow granted him the powers to do so.
Part 6 was an asspull however. Pucci actually achieved heaven and somehow in a world where he reset everything Emporio still remembered everything for whatever reason, and for whatever reason Weather Report's stand disk was also conveniently there, in a world where Weather Report was never a stand user. If that's not an asspull i don't know what is.
Part 7 ended with Johhny maturing from the edgy teenager he was for most of the part, he got rid of his self loathing attitude and depression, Gyro did his arc way back with Ringo, Lucy and Stephen Steel were happy together, Dio was killed, Valentine was defeated, and to come full circle, the kid Gyro wanted to save died from a cold.
Part 8 could have a nice ending if Araki doesn't fuck up, and honestly so far it's been good and i trust he won't go full Stone Ocean on this one.