Gonna go with Kingdom Hearts here for the simple reason that Oblivion had some big design flaws. Specifically, it was possible to REALLY fuck up your character in a major, unfixable way if you played wrong.
Example. I went the rogue route and stumbled across the Hitman-type side quest for the assassin's guild. The vast majority of that quest involved a huge amount of sneaking around. Sneaking gave you experience, and since that was a core ability of my class, it leveled me. A lot.
However, that quest also didn't involve any combat, so my level increase while my combat abilities did not. The moment I finished with that side quest, I went back to the main quest and found it absolutely impossible to continue because the bad guys had scaled to my current experience level, while my combat abilities had not. Simply put, I got owned by everything and there was nothing I could do about it.
So... yeah. Kingdom Hearts all the way here. I'd have voted for pretty much anything over Oblivion, though.