I've finished Steel Ball Run, it's my favorite part but Joseph Joestar still remains the best Jojo to me.
You can read Steel Ball Run without any prior knowledge of the previous series.
In Vento Aureo/Golden Wind I hate that all La Squadra members were introduced suddenly and all at once. I hadn't the time to be used to them, they brought all of them in the cast in only one episode.
While in Stardust Crusaders every member had a special episode dedicated and I was already used to him when he was about to join the main cast.
Also Giorno is quite a bland character, but not in the Jotaro way, Jotaro at least is cool while Giorno not even that.
Anyway, here are my rankings
Best Jojo (I still have to read part 8 Jojolion):
1) Joseph Joestar
2) Josuke Higashikata
3) Jotaro Kujo (Jotaro vs D'Arby senior is one of my favorites fights)
4) Jonathan Joestar / Johnny Joestar
6) Jolyne Kujo
7) Giorno Giovanna
Best Part:
1) Steel Ball Run
2) Diamond is Unbreakable
3) Battle Tendency / Stardust Crusaders
5) Stone Ocean / Vento Aureo
7) Phantom Blood
Best villain:
1) Yoshikage Kira
2) Funny Valentine
3) Dio Brando
4) Enrico Pucci
5) Diavolo
6) Kars (taken alone he is not that special, but in group with others Pillar Men he becomes awesome)
Best Jobro (divided by series):
Phantom Blood: Speedwagon
Battle Tendency: Rudol Von Stroheim(best Nazi ever, the only one you would like)
Stardust Crusaders: Polnareff
Diamond is Unbreakable: Rohan Kishibe (though he hates Josuke)
Vento Aureo: Mista
Stone Ocean: No remarkable Jobro, uhhmm if I have to say one, let's say Hermes
Steel Ball Run: Gyro Zeppeli
As you can see I'm really get into the Jojoverse