FF1: a bit difficult, and not much story, but fun regardless(save often!)
FF2: This one is a bit annoying, as it does not use level, but random boost from skill using(Using spell raise INT and MP (and spell level) but lower STR for exemple. It makes "levelling" anoying. Play only if you are a bit hardcore and completionist. It's the same system used in the gameboy ones(Legends 1-2-3), although the gameboy one worked better for some reasons.
FF3: This one is fun, the first using a job system(also used in FF5-X(2) also.) It's pretty good one for the NES or the remake on the DS. Can be a bit difficult at times but still alright.
FF4: Decent game, but there is something(that I still can't put my finger on) that never made me love the game. Finished it but, always kind of annoyed at the end.
FF5: Great game, using the job system. This one is just fun when you just play to finish, and as quite a couple sidequests too.
FF6: For me, the best of the best. I love Kefka, he steals the show. Your characters are all nice too, each with totally different abilities. Awsome music scores too. Actually easily the best musical score of all the FF. Others have good ones, but FF6 as the most memorable one I think.
FF7: Great game also, a bit long sometimes, but I just love the battle system. I never got bored of it(Unlike with some other FF.) The Simple 3d graphics are fun to look at too I think. Only thing that always bothered me is, no armor. Only upgrades comes from new weapons, bracers and leveling your char or materials. Materials are an interresting concept though. Good story also.
FF8: This one, is the only game where NOT leveling = EASIER. That alone makes it not so fun.(Nothing like one shotting a lvl 10 omega with a single Lionheart. What a disapointment). And also, NOT leveling is very easy to do, since boss gives no XP, and it's easy to run or card everything. The triad game though is alot of fun for me, but that's only a mini/side game. Two things killed the game for me though: The sidequests aren't that fun, gives very poor rewards most of the time, or are very cryptic without a guide. Other thing is they are very easy to miss. Second thing that killed it is the litttle chocobo herlper game. It just gives you all the items to have the best setup like at the first part of CD2(I think) whenever you can first get to a chocobo forest. Sadly, makes things totally too easy(add in the no leveling and .. yeah no fun).
FF9: This game, play LAST. Play all FF 1 to 8, THEN play 9, and if you have any sense of nostalgia, 9 will be an incredibly great and awesome game(Specially after the little sideway FF7 and 8 took). It's so filled with nostalgia and spoof from the previous games(specially FF1), it made it awesome for me. Also the gear/skill learning in this game is pretty nice(They ended up recycling it for FFTactic Advance and FFTA2. AWesome. But play it last, not first.
FFX: Very linear game until near the very end. Lots of interresting things. The internationnal verion may be more fun with the bonus boss fights, but *shrug*. It's great visually, but the Battle systems ended up pissing me off. Having to switch to all your char every fight so everyone gets xp gets boring very fast.
FFX-2: I played this one before X actually.(I though it was a different game heh) I liked it alot personall. It's very chearry and happy and all that. Not so dark or serious. Has New game +(always nice to have), and use the Job System. You can play this first without problem(Before X more specificly). I liked it.
FFXII(11 is mmo). I hated this one. Mostly because of the chain things, kinda forcing you to skip most to keep your chain going, which is really the only way to get drops for money(and only source of money most of them time). And that is VERY anoying, specially with the weird realtime based combat system(and your 2 ally usually doesn't help you with that, attacking anything in sight). Otherwise, not that difficult if you just finish the game, it,s lenghy though, and I never even felt like doing the side quests(unlke ALL other FF). If that sais anything.
FFXIII: Never played, all I heard was: it's a single path from start to finish, no towns, the end. Doesn't sound too fun. :/
Recommendations: Start with FF1(Specially the GB remake, uses MP instead of Charges for magic, but both are good. GB has sidequests though) or 6(Pure awesomeness), or even 7(Very symplified item list over all, might be easier if it's your first ever JRPG).
Second choices would be: X, X-2, 4 or 5. (All nice).
Play last: 9(Not because it's not fun, it's my second favorite after 6(easily beats 7 in my book), but because it gets even more fun the more you know the other FF I think). and 3. I think 5 is more fun then 3 for a job system based FF.(X-2 story fits in with X so it's a bit difficult for me to split those 2).
Skip unless you want to play the all: 2, 8 and 12.
There I type too much