Final Fantasy, as a series, is different for everyone. I was one of the very few North American gamers who's first Final Fantasy was the first, on the NES. Thief is totally the best, by the way. Now if only Square would make a new FF where you get to choose the classes of your party without a job system, that's would be great.
I feel everybody has one Final Fantasy they connect with. With most people, this was Final Fantasy 7. To anyone who played the series before 7, its universally agreed that 6 was the best. I'm a fan of 6 no doubt (5 makes a close second, but the music pales in comparison), and that's because it has one of the best music scores in video games that 7 just didn't quite get. I've played 6 over and over, and anyone can agree its dated and pretty average for a game, I keep coming back because the music just hits home so well. The main theme throughout the series is Terra's Theme, which gets played as you wander the world map.
The game has something like 13 characters you can have in your party, which is a lot, but not overdose like Chrono Cross. They also emphasize using each member equally, which I find really breaks the repetition, because in FF7 you can pick 3 members and play the game like that, and woop-dee-doo I'm bored by the Golden Saucer. What really shines is everyone's theme, each one is just an amazing piece of music, and if you don't mind I'm posting a handful of songs.
Each one captivates the characters well. Locke, the thief--Er, I mean, "treasure hunter" fights for a woman he lost, and has a very inspirational piece that gives that feeling of "keep on fighting, things will look up eventually", while still keeping a theme of adventure.
Celes has a much somber theme. One more of romance, desperation, and sadness, which I find strikingly similar to the scene where Aeris dies (SPOILER!!!). A lot of people call that the best Final Fantasy scene, other will say the Opera Scene in 6 where this song plays. The sad romance of a woman who lost her loved one in war, yet the song works just as well when Celes loses everything and drives herself to suicide via cliff.
And lastly, the windswept tune of a warrior who lost his wife, son, and entire castle to a psycho who poisoned the water supply. A song of wandering loneliness really gives you a feel for the poor man. Now if only the translation didn't turn him into a comic relief character 10 minutes later, they could have captured emotions better. But eh. Take it for what it is, a really good song.
Thanks for listening!