I don't really understand why Z is so popular. Its plot is an EXTREMELY simple tale of a guy destroying the world and the Straw Hats going on a chase to stop him. The Straw Hats were pretty much irrelevant as well as nearly its entire focus was on the Marines. Sure, Aokiji and all those Marines making cameos were cool, the animation was amazing, and the battle between Z and Kizaru was awesome. But it didn't seem like a fully fleshed-out movie to me at all.
It's quite obvious why so many people consider it the best movie, it's because the villain was well developed and somewhat sympathetic. And not only Zephyr's personalty shined through the movie, Ains personality shined as well, with her inner conflicts regarding her master's plans. Most people are tired of seeing Strawhats having fun and going through an adventure for some reason, if you look closely you will notice people getting hyped every time a chapter is revolving around world building, world government, world history, and anything not directly related to the Strawhats' adventure. That's why the heavy inclusion of the Marines (and Aokiji) made everybody excited for Film Z, also the series itself had a lot of hype back then because of the relatively recent timeskip, which made people want to see the new abilities of the Strawhats showcased. I can for sure say that many people liked Film Z more even before it was released, discussions about it was everywhere, people were asking whether it was canon or not, and all of these were months before its release date. Actually, I liked Strong World more, it was more playful and showcased the Strawhats very well, it was also structured like a normal One Piece arc with an interesting location (Z had these forgettable end-points that looked very generic), and had civilians you could care about (I bet many people forgot about the existence of the civilians who gave the Strawhats their outfits), and even the backstory of Shiki and his plans were more interesting than Zephyr's, I also didn't like how Luffy's fighting style was underwhelming (he uses normal gatling and not elephant gun) . I can't deny Film Z being more beautifully animated and directed though. For some reason I can't skip lines, which makes my posts look messy, I'm sorry.