I voted for Ultimas, but I'm not familiar with too many computer games.
As for Zork, isn't its gameplay similar to the 100+ digital comic/adventure/dating games already on PCE? I'm guessing that Return to Zork for PC-FX doesn't require a keyboard. Wouldn't straight ports of Ultima require text input? No matter what, even if faithful non-upgraded ports are what some people have in mind, I'm guessing that some sort of compromising will have to be made.
Doesn't Wolfenstein 3D for SNES run without any special chips? A perfect port might not be possible for PCE, but a custom version should be possible for any hardware. It just depends on how much needs to be altered vs how worthwhile the endcresult would be. The Genesis did a decent job pulling off a few real time fps games.
Other: Mysterious Song!
Maybe if you upgraded the gameplay, menus, music, battle bgs, rest of the tiles and sprites, extended the game and piled on a bunch of special features... But that would be a lot of work.