Steel Ball Run absolutely deserves the title of "Best JoJo Part". The other parts of JoJo have numerous flaws, and even though I love them to death I am more than willing to admit that Araki is not infallible. His manga is not usually thought out ahead of time, and often it's fairly obvious that he didn't have a conclusion in mind when he writes. Sometimes this works, and other times it doesn't work as well. But Steel Ball Run is such a tight narrative that, with such amazingly written characters and such an enthralling story that I'm not sure I can name any flaws.
Araki's art is magnificent here, his paneling is arguably at its best, and his character designs are as wonderfully unique as ever. The writing stands out as especially great. While in other parts some characters would get last minute backstories, or rushed development, Steel Ball Run makes sure to introduce characters and motivations naturally and realistically, and this makes the world feel all the more real.
Going beyond just story, the fights are fantastic in this part. I'd argue that Stone Ocean is the low point when it comes to the action in JoJo, but in Steel Ball Run every fight is action packed and interesting. All around, this part of JoJo is simply perfect, and that is not a term I throw around lightly but I genuinely can't think of any negatives about this part. Truly Araki at his best.