Oh man. A Final Fantasy thread...you have just made my life proulxski.
I voted for FF IX as my favorite only because it was a huge throwback to the original FF's that I played, I and III/VI. I'd have to say that VII is probably right up there as well...that game is quite possibly the greatest game ever created in my eyes. The plot of that game is absolutely mind-numbing. And Advent Children kicks all sorts of ass.
I agree Nobuo is a genius...I just got the OST to Advent Children, and it's amazing. It's all the VII songs but reworked for REAL instruments, not MIDI. And also the concert that Nobuo had on 02/20/2002 with an orchestra for all his FF songs is WILD. My friend has it, not sure where he got it from, but it's awesome.
VIII kicks ass too, effe what the reviews said.
I can't wait for XII to come out...the screenshots and things I've seen for it seem astounding. The scenery and colors and stuff is so vibrant, so animated. The FF series is so good at creating a perfect atmosphere for their games (VI particularly, after you kill Atma Weapon and the world "ends," I've never felt so dismal from a video game ever).
/rant