Hmm, I'm really not sure which one I'd vote for!
It's a toss up between 7 and 10 for me.
The scale of 7 when it was released was epic, there was nothing else like it. It was the first Final Fantasy game I ever played, as I'm sure is probably the case for most others in this country. The story was really well done and the twist was brilliant.
I also really liked 10, the tropical environment and theme to it I thought was great; coupled with the updated PS2 graphics and proper speech. It really brought the story to life so much more. I agree, the dialogue was shoddy in places, but they did a good job of it, and really there weren't too many previous games that had been so dialogue centric.
I've not been amazed by many of the other FF games, and the new one I've not bothered with. Something about the trailers I saw for it didn't really do it for me, also playing a FF game is a fair investment of time!
Really can't decide which I'd pick though.
I think 7 would have been even better had it been made during the PS2 time.