For Ds i think there is only FF3 that has been released right now but FF12: Revenant Wings is still to be released.

As for Ps2 there are a number of choices: FF12, 10, 10-2, 11 ( If you can play online and pay )

My favourite StoryWise is FF10 but GameplayWise is FF12, You could also go oldschool and get FF7,8,9 (ps1,pc( 78 )) and there are remakes of 1+2,4,5,6 for GBA but you could get those another way without buying *Cough* ;)

Caution MEGA WALL OF TEXT APPROCHING!

Major Edit:

From the FF's i have played its very hard to decide which is the best:

FF1+2 ( They come togethor ) - 8/10 Was the original :D Good gameplay but 2 was a little more difficult to get a grasp of.

FF4 - 7/10 - This is the only FF i can think of that i have not fully completed in terms of side quests, etc. None-The less great game.

FF5 - 9/10 - One of my fav's, some of the best characters and gameplay mixed with the great story.

FF6 - 10/10 - Right now my Second fav. Much more characters than i have ever seen who are playable Inlcuding a MOOGLE! Fixed jobs but everybody can learn magic, etc from Espers and there is something like 20-30 espers.

FF7 - 10/10 - The best, Story, Gameplay, Characters, Side-quests. Overall the best!

FF8 - 7/10 - Not in the top 5 but still a decent game, had a different approach in terms of the Junction System and wasnt all it was ment to be.

FF9 - 9/10 - One of the greats. Awsome story and gameplay with characters you love. It had a fixed job system style.

-------All the above FF's used the ATB battle system-------

FF10 - 10/10 - A masterpeice on the Ps2, revolutionery graphics, gameplay and characters. Even though some people didnt like Tidus i thought he was in the top 10 best FF characters in my Opinion. In the begining the characters are fixed jobs, but toward the end and after some extra battling you could learn the full Sphere Grid. You dont level up neccesserily in this game but your Sphere Grid levels up in order for you to progress on the board learning more abilities.

-------FF10 used the CTB ( Conditional Turn Based Battle )-------

FF11 - N/A - Havnt played it, dont have online 360/ps2 and pc cant handle it :(

-------FF11 used the RTB ( Real Time battle )-------

FF12 - 9/10 - Again some people hate it some people love it, i love it. Characters werent the best, heck to be honest there were some that were just plain Stupid ( Vaan :P ) It used the Licence board system that was a upgraded version of the Sphere Grid System that was used in FF10 ( If develops your characters in terms of what the can learn, magic-weapons, etc) FF12's character development isnt the deepest but it still good in my opinion.

-------FF12 used the ADB ( Active Dimension Battle )-------

Every FF with the exeption of 10 Involves you leveling up to a max of 99 except FF8 were you get to lvl 100.