A while ago, I would have said Days. But I can enjoy Days for what it is. Union X (and by extension Dark Road) is in much more dire need of it to me. Days is at least a full and honest game with a well-told story. Union X and Dark Road HAVE good stories, but they're buried beneath unforgivably bad game mechanics, relentless filler in the case of UX, and horrendous grinding in the case of DR. If you made me choose between one of these two, I'd probably pick Dark Road, only because I think Xehanort is more of an important character who deserves to have his story told in a respectable manner, and not in a cheap gatcha game with chibi graphics. But ideally, I'd have them both remade from the ground up.