As you've certainly heard me say before, "gameplay is the most important quality of a game". Whether there's truth to that or not, there certainly is truth for it in my own preferences. I enjoy all the fightin' in action-adventure games like nothin' else. As such, I especially enjoy Kingdom Hearts' combat. I've also a strange knack for blockin' out the bad qualities of a game that have nothin' to do with gameplay mechanics; poor stories and repetitiveness in particular for whatever unknown reason (ohai my love for Assassin's Creed 1).
In short, the gameplay. It's not deep or all-that challenging often enough but I still enjoy it simply because. Ah, and Yoko Shimomura too.