I will defend FF8 to my dying breath because it was my first exposure to both Final Fantasy and the PlayStation in general and it was amazing and fuck anybody who says different GRAAAAAHHHH...
... ehem.
Anyway, I haven't played FF13 yet, mainly because a lot of the newer stuff I want to hold off on until I get an HDTV. My friend got a 55" HDTV with his income taxes (after being a responsible adult and getting out of debt first,

for him) and, well, it's kinda spoiled me, so I've been sticking to my older stuff a lot lately.
I didn't play much of 12 because I went through a bit of a "fuck RPGs" phase that I am just now coming out of (points to his Backloggery), but what I played was pretty fun.
As far as the PS1 FF's go, I don't really have a clear-cut, 100%, "I will play this before I play the other two" winner. They all have their merits.