Now my inner fanboy tells me I should vote for FFVI cause it's my favorite and cause I feel it's cinematic enough to be remade without major alterations but deep down I know that FFIV deserves it. Flesh out the characters and their backgrounds, seeing the dwarven kingdom or the Lunar caves with FFXII graphics. It's a game that gets nailed alot cause it hasn't aged well but a revamp could fix it. I'm not sure what to do about the battle system though. I doubt it can be left the way it is, but I can't think of a system that work for the best with this game.
FFVII doesn't deserve one yet. At least not until they talk about fixing the horrible writing and balance issues that prop up in the game when they do a remake. I'll admit that AC and Last Order have fixed a few issues (Last Order especially, since it completely changed a scene I've been complaining about since the game first came out.) but there is still alot of better FF's that could use a remake before it. At least not until they remake FFIV-VI.