Final Fantasy VII is by far the most popular. VII, VIII, and IX were Sakaguchi and Uematsu's apex. When Enix bought Square, X started the decline, and they both left after that. IMO the new games are alright, but they're not 'Final Fantasy' good.
RPG are about the story, and FFVII is just a really great story. I mean, I could tell you the story, but I'd hate to ruin it for you.
Incidentally, Sakaguchi (the designer) and Uematsu (the composer) now have their own company, Mistwalker, and they're making some really good stuff. Like The Last Story that I mentioned.
Also, Nobuo Uematsu's music is independently phenomenal.