I think Final Fantasy series is the best.

I do play other games besides FFs, but.. I can play them for hours unlike some other games, like.. sports. XD
Anyways... I can't pick up the BEST of the 7 FF games I have, because I can always find something good about them.

So, I'll just list them all here and tell you guys what I think about them. ^^
VI: This one I'm currently trying to finish, I need to beat at least 1 Dragon and train my characters a bit more till I can go to Kefka's Tower and try to beat him up. The graphics part look like... ehh.... well, like a Nintendo game from 1998, ahahaha, anyways, even though it's not so great the rest of the game makes it awesome and fun to play: the plot, characters and some certain events make me laugh and cry (and yell) very often. The Magic system is little bit weird, you need Espers to learn Magic, and that kinda sucks, because if you miss one Esper you might miss a good spell or two (I don't have Flare nor Ultima... -___-). This was my 4th FF game.
VII: This was - not so surprisingly - my 1st touch to this amazing game series. I saw this lilla boy playing VII, tried it, wanted it - and went to the store to buy it and became ill the same day. Such a coincidence! Anyways.. I've played VII through at least 3 times, and I seem to find something new about it everytime I start a new game. XD As many have probably said already, the graphics are not soooo good, but again, the plot with its twists and turns, characters and humor swallows you in. Actually, even though Midgar is a dangerous, dirty and nasty place, I'd be happy if I ever had a chance to go there. XD XD *laughs at self*
What else I liked about this game? Well, the Materia system idea sounds great to me, and that your Magical skills (attacking, restoring, protecting and supporting magic) depend on how many Materia slots your weapon has. MMm, I wouldn't mind if the rest of the FFs had the same system! XD
VIII: This was my 2nd touch on FFs. I was a bit disappointed with the Menu screen (GREY!!! and no chance to change it..!), and sometimes the female characters just made me go mad because they were ..annoying. =D Still, I enjoyed the game very much; the graphics were better than on VII, and when a video clip appeared in the game, I stared at the TV screen eyes wide open and if someone dared to disturb me I started to scream curses at them. The plot made me laugh many times (Rinoa and Squall were too.. well, predictable! Yet, their story was quite touching at times..), and especially Zell received my sympathies. Sorceress Edea is still my nro 1 favorite of VII characters. The Magic and Summoning system was clearly different from VII, but I learnt it quite fast, and I soon relied a lot on the Guardian Forces, using them everytime I had the chance!
IX: My 3rd FF game. I haven't finished this one either, not yet, but after VI I'll focus on this one with full concentration! Luckily I have a good save to continue on so I don't have to restart the whole game... XD Anyways, this is pretty much different than technological VI, VII and VIII, IX's word is soft, pretty - yet it has its dark places (Evil Forest, for example). The plot is the usual Let's-save-the-world, yet IX - like the other FFs - has some certain events that make it different from others. =) Garnet & Zidane were not a surprise, and Vivi received my sympathies, and Steiner and the Q-guy (what on earth was his/her name again xD) make me laugh a lot! The Magic and Summon system was again different than in VII (or in VIII) but it's quite easy to catch up with. I don't like the weapon system, because when missing an item or a weapon, you can't Synthesize a new one to make your attacks stronger. =/
X: My 5th FF and 1st PS2 game. Man I was surprised when I first played this! Everything was so different from the previous games, and I thought I'd never be able to finish this one! But I did, and I'm still surprised! The graphics are just amazing, the plot is really interesting (yet not very different than the other plots, haha) and touching, and the characters! At times I was thinking that I was actually watching a movie and at times I was able to do something, like move them around and fight against the fiends, and confirm some stuff. I really hated Blitzball, probably because I can't get the idea (of any ballgames), and the Sphere Grid system was really confusing at first, but I really loved the Summoning system, the Aeons. =) I think they are the best Summons. ^^ I also liked the fighting system - man, you can change your party while battling! I'm always angry when I've played X then play VII (for example) and I find out I can't change the party during a battle, eheheh.
X-2: Now who was surprised to see that the story of X was to be continued? Well, not me. What surprised me the most - and annoyed, too - was the fact I can only use 3 girls that are mostly half-naked when having a certain, here-unnamed Dressphere, and that everyone can GUESS the end before even opening the game. I haven't finished this one, either, but shall concentrate on this after VI and IX. Even though the plot is more than just predictable, there are events that make me laugh and cry, and the song, "1 000 Words", is one of the most beautiful songs I've ever heard. The Dresspheres are also interesting, but at times they really make me mad and frustrated (like, White Mage, who can't fight at all (I like quick fighting more than healing, haha)). My 6th FF and 3rd game on PS2 console (a non-Square game was the 2nd).
XII: Who is surprised to hear I have this game? Well, I bet you're all surprised to hear I have not played this game yet. YES. It's on my bookshelf and I'm not playing it. WHY? Because, if I start XII now, I'll never finish the other games! =O So, I'm (impatiently) waiting, because I've heard this is amazing stuff and a bit more challenging than some other FFs, and omfg, I paid 65€ to get this! *.* So... I'll eat my ice cream and hurry to play VI soon, so I can get through IX and X-2 so I can FINAL(LY) start XII! (And this is obviously the 7th FF I have. =D )
Wow, such a long post, and it's my first. =O Eh... I wonder if someone even reads this... XD
Oh yeah, my father had IV and V, but to get them for myself I would've paid 10€/game. :| Not fair from my dad... But if he still has them, I'll "buy" them after getting some money from my summer job... because I love Final Fantasy! =D *laughs*