It'll always be KH2's opening for me. The storytelling, the storyboards, and the music is all so incredibly good. KH0.2 is a close second for similar reasons. I really liked KH3D's opening too while KH3's opening felt a bit to similar to 3D's opening. I also didn't like the whole chessboard thing too much. And I also got a bit tired of seeing the whole series being recapped, again. Understandable as KH3 serves as a conclusion for all those previous games, but going forward I hope future openings are a bit more abstract and spends less time lingering on the past.
The ones I like the least are KH1's opening and BbS' opening. They're just too bland for my taste. KH1 is quite nostalgic at least and a bit ominous while BbS' opening is too visually boring for my taste and uses way too many scenes from KH2FM's secret ending.