Avoid any of those newer than #9... They're just shitstorms waiting to happen, and they seem to be more about glamor and pazazz than actually coherent stories and gameplay that doesn't suck a big fat one >.< I recently tried playing 13... couldn't make it beyond the first five hours without getting bored out of my skull.
Numbers 6, 7, and 9 are about the best ones in my opinion, #4 in a pinch. 4 and 6 you can probably find for under $20 in their remade formats on the PS1, and 7 & 9 are also for that same system. Another suggestion would be Final Fantasy Tactics Advance for the GBA; decent story, fast-paced combat (for a strategy RPG, at least XD) and tolerable characters. Not sure about FFTA2, but I've not heard bad things about it either.
Were you thinking just about NeverendingFinal Fantasy, or other JRPGs as well? There's more than a few that I can think of that'd be easier on the mind than FF has been lately...