I'm generally not a big fan of remaking great games...great games are great regardless of what format they're in. I mean, part of what FFVII is is it's graphical style. VII and VIII, maybe even IX and X, could use a little tweaking (the character/monster graphics in particular), but they don't need extreme FFIIIDS-graphical overhauls IMO.

That said...

The game that would be improved most by a graphical update as long as the game itself was left intact is FFVI IMO. I agree with silentenigma that Kefka in high-def would rock. So would Vector, the opera, and lots of other parts in that game.

The game I think would stand to improve the most by being completely overhauled and remade is FFII. IMO, a great foundation for a game that was just not pulled off as well as it could have been (though perhaps pretty well for it's time). Really develop the characters, story, and locations, and redo the random encounters thing so that the battles are more interesting and less tedious and IMO the game could really shine.