I'd be fine with any of the 8-bit and 16-bit ones getting a Trials of Mana style remake in terms of visuals. Heck even IX could work with that.

VIII I think is eventually ripe for a VII style remake as the original devs have spoken quite a bit about the stuff they wanted to do with that game but either couldn't, or it had to be cut.

I imagine VI or VIII are the most likely candidates at this point. Although I won't be at all surprised if Crisis Core gets the Type-0 HD treatment.