FF9 is perhaps my favourite, though I'm influenced by the fact that it was the first that I played. Gotta have that nostalgia value.I enjoyed 8 and 10 though, and I'm probably one of the few people in the world who actually enjoyed FFX-2. It was a fun non-linear experience, and it was interesting to go back and see what changed in Spira since the original game. Part of me often wondered how things would turn out after the game ended, and FFX-2 more or less allowed me to see that. FF8, FF9 and FF10 easily stand up as the three games in the franchise that left the biggest impact with me. Lot of halcyon memories of being in high school and playing through FF10. 

This is where I probably detour the most from popular opinion: I played some of FF7 but it just didn't resonate with me, I couldn't get into it now matter how hard I tried to, which surprises me even today since I like the themes and aesthetics of FF7, the whole bleak cyberpunk future and all. Many would list FF7 as their favourite title of the PlayStation generation, though for me that accolade falls to Resident Evil 2.