To start off, played the following: FF VII, FF IX, FF X, FF X-2, FF XII, FF XIII, FF Crisis Core, FF Dissidia
1) FF X - The time I waited to get a PS2 after I already bought the game, story and levelling system and the fact that I have invested the most time into full completion wins it. The combat was also one of the best. I enjoyed that when I opted to do something, it actually happened as well. Not like with the regular ATB, when there can still be something queued up by the enemy even if your own bar is full and action inputted (FF X-2 really had the best ATB system imo). Knowing who and when was going to perform an action gave a more strategized point of view.
2) FF IX - The first FF that I played. The first RPG I played. The game that got me into RPGs. Benefits: again, interesting story. Played through it 4 times. Enjoyed the fact, that every character was fully distinguishable from another, not only because of looks and special attacks, but from the base skill set.
3) Difficult. I would actually say either Dissidia or Crisis Core or FF VII. Dissidia is pretty much my favourite fighting game ever and it's quite upsetting that it's a PSP only game. The story is well...not really something I paid attention to. But the amount of fun to be had here is limitless. Escalate the fun with Duodecim, which has quite a few improvements, while the core game stayed similar and it definitely feels like a winner.
Crisis Core gave a slightly refreshing perspective on the FF VII universe. While some parts in the story feel ridiculous after finishing, then the upsides greatly outweigh the bad parts. I quite enjoyed the combat, and I still listen to the soundtrack.
FF VII itself is simply a classic. The gameplay was quite varied, and the customization of the characters was on a high level. Plus there was a ton of stuff to do. Even if the Gold Saucer wasn't my favourite pastime, it was still there. And it had a lot to do outside of the general story or gameplay mechanics.