The emotional heart of VI is with Celes at the beginning of the WoR. I love VIIR as a game but I don't love the story changes, and the one that's relevant to a VI remake is how they handled Dyne. I honestly don't trust them to keep that crucial moment in VI intact.

This is probably controversial but I think X is the classic FF that's dated the worst, with an the dodgy voice acting, poor character animation and the like. It's in that awkward point between classic and modern and doesn't really do either well by today's standards. Also, no world map or exploration. It's a beautiful game in most respects but I think has the most room for improvement.