From the number of replays and how much I like the gameplay: FF X. The story has a few plotholes but overall its well done. 6-9 are close seconds. Didn't like the series much after FFX though.
7-10 are the best imo. My personal favorite is 7. Everything just lined up perfectly for it. I loved the characters and the story. I really like the materia system. All of that with the timing of when it came out. I've probably bought at least 4 or 5 copies of it across all of the systems it is available on.