This is a tough one.
Part of me wants to tell you to stay the hell away from Final Fantasy. The fanbase is pretty awful, to be honest. There seems to be very little room for respectful disagreement and a lot of childish, "OMG HOW CAN YOU LIKE THAT ONE IT SUCKS WTF." It gets bloody tiresome. Some fans just want FF7 repeated forever, while others will have a baby cow if there is anything different from the formula--while complaining about the formula.
Remember, this is one of the few games where a large part of the fanbase doesn't even like the genre itself.
All you have to do is read some of the whining and crying about FF XIII. I thought it was a good game. Not one of the best Final Fantasy titles but still good. I enjoyed playing it and I liked some of the character, though some were annoying. However, if you listen to the fits people pitch over it, you'd think it was the very definition of evil.
The fanbase isn't as bad as the DMC or Sonic fanbases but it's still toxic at time.
Now, with that out of the way, I will tell you that I think FF6 is the best Final Fantasy main-line game. It's the very definition of a classic RPG. However, Final Fantasy changed with FF7 to become more cinematic, more stylish, and more nonsensical in terms of story. That's really the dividing line between the new and old Final Fantasy, so FF6 isn't exactly the most representative any longer.
With that in mind, I'd probably recommend FF10. It's held up well, it has some good characters, good gameplay, and the story isn't the worst thing in the world. FF9 is another really good game but I'm not sure how well it's held up. It's almost more of a throw-back game, as well, having little to do with the modern era Visual Kei feel.
