Not 8 (or VIII or whatever)
I've played on NES the original FF1.
And on PSX, I have played FF5+6+7+8+9.
FF8 was disappointing - especially after how great FF7 was (i.e. the one people cite as one of the best FF's namely). I didn't like FF8's huge emphasis on constantly using Summons (those can be long-winded and might give you the feeling they do a lot of the work for you if you don't master how to Junction); Junction system wasn't my favorite system; Drawing (i.e. stealing) Spells (to learn them and allows more uses of that spell in your pool) was cool but shouldn't have been the main focus on how to learn & use magic; Squall is way too annoying & whiny for a lead (for most of the game); and the story stuff falls apart and is a mess after the first Act and the first awesome plot twist in the story; and the build-up for the final villain is a complete waste by the end of the story.
I loved FF6+7+9 - to me, these are FF at its best. Really fun turn-based combat (these are usually active time-based and/or a turn-based system) with great story & characters. Personally, for me: FF6 is my favorite FF.
FF5 is really good b/c of the Job class system (i.e. learn multiple character class roles), but its story & characters just aren't my favorite FF. It's still good - but I prefer FF6+7+9.
I liked FF13 (Part 1), but it's a much different FF game. So, keep that in mind when going in, if you plan to play. Still ain't finished it up - but for the first 15 hours or so that I played, it's very linear & there really ain't any towns. Supposedly, it does open up a bit later - ain't hit that point yet.
It feels very heavy w/ dungeon-crawling with FF-style combat . Since there's no towns, you basically hit-up Vending Machines (which also are save spots). You have a cool job-like system (called Paradigms) that you switch on-the-fly and characters slink into different roles when in combat. To me, this is the real fun of the game - this combat system's a blast.
There are lots of characters. FF13-1 constantly forces you with using different characters for different chapters of the story - really keeps the gameplay, combat, and story fresh. I really do love the story, characters, & combat system - once this stuff actually really gets going some 5 hours into the game (i.e. when they introduce Paradigms). You probably should read the Codex for Summaries of each Chapter & to learn the game's lingo (b/c it uses a lot of its own terms and you will need to know who is who and what is what), just to make things a lot easier & clearer for you.
Also, the game is just down-right beautiful artistically. FF games never seem to disappoint in this regard.