My hardcore final fantasy history may help you understand a little insight on why they were so good.
First off FF games are known for their story, music, battle system, great cast of playable characters, graphics (at the time of release) monster summons, world exploration and a fantasy universe make these games so memorable. Oh and usually over 100 hours of gameplay if you do everything so you really get your money worth.
The best Final Fantasy in my opinion is FF7. It was the first PS1 game I owned, first FF I played, and I still consider it to be my favorite game all-time. I'd say it got me hooked on video-games and made me the gamer I still am today. I was absolutely blown away. I remember being 40 hours into disc one and thinking OH MAN I still have 2 more discs... how long is this epic game!? The characters and story were unique and very likable and the the materia linking and battle system very engaging. In 1997 the game was also cutting edge in terms of graphics, FMVs and even had orchestrated music. I did everything there was to do in this game. Over 100 hours of gameplay here.
Final Fantasy 8 was a bit of a disappointment after all the hype 7 created. I raced through 8 in less than a week so that didn't help either. It was still a fun game. Keep in mind it was also cutting edge at the time in 1999. Magic combat in 8 was weak since you had to go around Drawing magic from all the enemies to just save because it made you not want to use them in order to boost your stats. All I did was attack and summon. Also you could get very far in the game without leveling up since enemies stayed around your lvl but bosses had a minimum so one of my friends got into trouble this way and couldn't finish the game. I had the opposite with trouble on regular enemies with everyone at lvl 100. I did everything there was to do in this game including collecting every card in the triple triad card game. Over 100 hours of gameplay here.
Final Fantasy 9 I felt came close to topping FF7 early on with the great cast of characters, but with a final boss in the game that you didn't get to know or care about (same as 8) hurt the game a lot. Seems to be someone higher up pulling strings. No FF boss has been as evil, foreboding, and persistent throughout the entire game as Sephiroth in FF7. Going back to the series roots of more fantasy cartoon-like characters was a charming touch that a lot of people like about the series. Also the combat using weapons to permanently learn abilities through them was excellent. I did almost everything there was to do in this game. I know I missed one of the optional bosses in the library of the town that gets destroyed by bahamut. I need to go back and play it! Put over 100 hours here too.
FFX and FFX-2 and FFXII attempted to follow the same recipe but got worse with each one for me.
They lost the over-world map exploration and had a linear map that everyone followed. I still enjoyed 10 for it's story and battle system, a true turned based combat where enemies wouldn't attack until it was there turn. This made for a lot more strategy and planning of each attack which was very cool. Controlling monster summons were also pretty cool. I also liked the story with Sin being a persistent enemy, 2nd best to Sephiroth IMO. The switch to voice acting actually hurt a bit and took away from the imagination. Auron was good though. I did everything there was to do in this game 'got the one who conquered all' and filled up my battle grid completely with max 255 stats on a lot of stuff. 50 hours to beat and over 100 hours of gameplay to get everything.
FFX-2 was the least appealing to me with the girl power overtones. The combat system was still fun and you could stop an enemy from attacking you mid-animation if you were quicker to hit attack. Weak story really hurt though. I think I did manage to make it to 98% complete with about 65 hours so it was on the short side too.
FFXII was also disappointing to me. Cast of characters was kind of generic. Never did I want or feel the need to do anything but hit "X" to attack (and pause to heal) I'd let my characters automatically attack by assigning gambits. The game was on auto-pilot for me and battles just end so much faster with regular attacks than going through a command list. I think it was my least enjoyable final fantasy.
I beat the game but did not do all the extra hunter quests. I did get to fight Yojimbo and his dog though which is pretty far into the quests. Can't remember how many hours I put in.
I have high hopes for a return to glory with Final Fantasy XIII but for me the series has been on a downward spiral since FF9. I always worried that the series may tire itself out with too many games but I'm still hopeful b/c each series is a final fantasy for a new set of characters. No game has ever had the same characters except for the spin-off sequel X-2.
Best in order for me:
FF7
FF9
FF8
FF10
FF10-2
FF12
Never was able to get into the previous games FF1-FF6. I tried the Anthology but the graphics gave me a headache and I just couldn't get into it. That's probably the case with many people for FF7, 8, and 9 today so perhaps its a good time to remake FF7 for a new generation of gamers! 
