Then nothing else matches. Titles like DDD not only feature immensely important chapters of the story, but have gameplay mechanics that are featured in subsequent games.
Yeah but it isn't the main story. Important, sure, but not really the main quest.
Okay, cool, thanks for proving that you haven't even played the games (or at very least haven't played them enough to know what the story is).
Dream Drop Distance is literally the next sequence of events after KH2 for Sora and Riku in which they progress the main plot forward. It's where Xehanort makes his next move following KH2. The idea that DDD isn't part of the main quest does more to show how unprepared you are to even have this conversation than to prove the point you're trying to make.