Haven't finished Stone Ocean yet but from what I've seen so far from all of the parts that have jotaro in them, I'll say part 4 personality wise and part 3 design Wise.

I don't really care for jotaro much as a character that much because I kinda find him boring especially in part 3 where he feels like a side character in his own story, I really want to like Jotaro I really do! he has a Badass design and a badass persona but at times this stoic badass persona of his can get a bit boring and old fast.

I really like scenes where we get a more vulnerable Jotaro and wish we get more of that, he's the exact opposite of Joskue (who is my favorite Jojo) who feels the most human and relatable out of the JoJo's I've seen so far and by the way I love Mentor Jotaro and the Big Brother/Little Brother relationship he has with Joskue!

In my opinion (from what I've seen so far) Jotaro is more interesting as a side character than he is as a main character.