I agree. Xions ending really hit hard. Believe it or not Aqua's is alot sadder.
There's a channel on Youtube called Strwbrymilk and she's a friend of mine, she subbed all the cutscenes on the JPN version of BBS in a perfect sequence to the three storys. First Terra then Ventus and finally Aqua. You should check it out. It's alot easier to watch them than I'm making it sound. Anyway, Aqua's ending and the secret ending almost made me pretty sad, and that's not a easy thing to do..haha.
Have you played Dream Drop Distance? Oh and are you into anime or Manga?