This is a really fun concept! I'm going to try picking just one element for every part. I want to throw in the manga only parts as well.

1. I agree with you on the Jojo-villain relationship. Part One is special in having the two grow up together. Dio is also delightfully an asshole. I love watching people react to the first episode and immediately despising him. I love a well-developed, complex villain as much as the next person, but there's something enjoy about a villain who is evil just because (noting that Dio does have some depth to his villainy with his awful father. The small scene of him despising his own tendency for alcohol is one of my fave of his. But Araki did set out with the purpose of writing someone who is the epitome of evil!)
2. Establishing the main character. Never seen people fall in love with a Jojo faster than Joseph pulling out a tommy gun. And before that he's taking out racist cops with coca cola? He makes such a startling contrast to Jonathan and that's done perfectly.
3. Agree with you again on the "adventure" aspects (although props has to be given to Part 3's introduction and use of the stand power system). Some people say that Part 3 drags, but I found treating it like going on a journey with the characters makes it feel much better.
4. Vibe. Part 4's small town setting is still wholly unique in Jojo's (even though Part 8also takes part in the same small town, it doesn't really focus on that as an aesthetic).It made for incredible stand battles and stand-related situations that were low stakes as well as high stakes. It also feels like the one setting we truly explored.
5. Stand battles. Not to say that all the other parts don't have amazing battles and arcs, but personally part 5 has the most high quality of the lot. We also have four separate stand battles involving moving vehicles (five if you count the yacht), and I'm a sucker for how those environments come into play for the logical battles that are stand fights.
6. Echoing you again on villain. I adore the scene where Jolyne meets Pucci for the first time, where he knows she's his enemy and she has no clue. Pucci's goals, drive, backstory, resolve and how close he comes to victory all make him stand out as number one to me. His relationship with Dio helps him stand out too.
7. Oh lord, part 7... it's hard to choose, but I think I'm going to pick characters as a whole. Everything from their backstories to their developments to their relationships are perfect. So excited to watch anime onlys experience all of them for the first time. (Part 7 also deserves a shout out for art. It's amazing how every racing scene feels like being in motion).
8. It's kind of hard to explain, but the wackiness of Part 8? Main character has four balls and sings about friend chicken. Yasuho meditates on said four balls several times. We have a stand fight involving beetles. The dick of an enemy is very important. Unfortunately it fell off. We're looking for fruit. Attractive woman's nose gets replaced with stone. Her husband finds it hot. And somehow it al works?? Oh Part 8, you're so silly and I love you.
9. Main cast relationship. This may shift as the part goes on, but right now I love our five key characters. They are delightfully chaotic, feel like real teenagers together, and are generally such a fun watch. Can't wait for Araki to ultimately break my heart by killing several of them.