Well, one method is to check out fanfic review blogs or websites. Some bloggers dedicated to JJK will review and rank fanfics. Also, ask other JJK fans directly. You can go to anime or manga conventions where there are likely to be JJK fans. They might be able to point you to some amazing fanfics they've read. Additionally, explore the related tags on fanfic platforms. Sometimes, the less popular but really good fanfics can be found this way.
Well, 'Yuji's Ascent' is a top - notch JJK fanfic. It follows Yuji's journey in a more personal and detailed way. He has to face new challenges and grow in different aspects. Then there's 'The Cursed Family'. This fanfic is interesting because it creates a whole new family dynamic within the JJK world. Also, 'Satoru's Redemption' is quite good. It gives Gojo a new arc where he has to deal with his past mistakes and strive for redemption.
I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.
The best scene could be the one that has the most character development. The scene where Vincent takes Mia out and they have all those interactions. It shows a different side of Vincent, who is usually this tough guy, and we see him being a bit more gentlemanly and nervous around Mia. This kind of character exploration within a scene is what can make it stand out as the best in the movie.
I'm not sure about legal places to read it for free with good translations. It's best to support the creators by buying or subscribing through official channels.