Meh, it don't matter on the graphics, it matters on the gameplay and ideas broski.
Lol, what Hillboy said: gameplay in an opening? But back to your point, I know that better graphics don't matter. KH1's opening had amazing graphics set to an amazing song as well. The ideas that opening set have reappeared throughout the series time and time again; darkness (the wave) falling over Riku, falling through a 'Dive Into the Heart', falling asleep, etc...
But KH2's recapped the series so far at that point. Also it gave us our very first in-game glimpse of Roxas. Namine was displayed in CGI. Etc...
Then comes BBS's. It basically foreshadowed everything that was bound to happen; the battle in the Keyblade Graveyard. Terra's fall into darkness(shown in the Castle Oblivion running scene), Ven's refuge in the light (Sora's heart/the light in the Castle Oblivion hallway), and then Aqua getting stuck behind and watching her friends fall, Ven's heart shattering (the stained glass mural is symbolic for this), Terra and Ven's hearts falling asleep (when their eyes close), and then Young Sora and Riku watching Ven (as a shooting star) on the horizon (symbolic for how TAV were going to affect their lives in the future/distance). Yadda, yadda...
I see KH1 and BBS's openings as symbolic videos while KH2's and DDD's are recaps. All four are amazing, IMO.