Although I've never watch any of the KnK movies, I've heard all the ending themes. And they're all beautiful. But naturally, being human, I have my favourites among them... XD and I think many people will too.
So if you'd like to, justify your choice, add second and third favourites, etc etc etc.
If I have to choose, I'd choose oblivious, the impactful first ever Kalafina song. The background vocals are beautiful, the harmonizing can totally derail my train of thought, and the song overall can make me feel better in general. Following quite closely behind would be fairytale, then Kizuato, also for the harmonizing. And the melody.
It was a tie between 君が光に変えて行く and Aria, but in the end, I went for Aria. Both really struck a personal chord for me on an emotional note, and the lyrics were so breath-taking. I chose Aria in the end because I prefer its higher-pitched(?) vocals - especially in the chorus - and the melancholic melody :3
At first, I admit, oblivious was my favorite. I mean, it was the first song and it opened the gate to getting deeper into their music. But now, I am really hooked with sprinter. The rock and orchestral merging, together with the vocals is NOT an easy job. People in the Kara no Kyoukai must be really happy to have Kajiura and these three to perform the theme songs.
2. Oblivious (actually on par with Sprinter, but Sprinter was the one to make me love Kalafina for the first time)
4. Seventh Heaven (heavenly goes to heaven)
5. fairytale (certainly brought into a fairytale with this song)
7. Kimi ga hikari ni kaete yuku
Seventh Heaven. At first, it was sprinter. I just love how the song kept changing itsself and how they used all the vocalists, but after I watched the last movie, I just CAN'T think there's a Kalafina song better than seventh heaven. Everything seemed perfectly planned to be perfect: the lyrics, the arrangement, the voices. Truly a masterpiece.
Seventh Heaven is easily my favorite album of theirs. I loved every bit of Kara no Kyoukai in its entirety. Never before did I enjoy a show so thoroughly.
But my favorite themes are Oblivious and then Sprinter, and then the rest - because these songs really defined the album and the Kalafina style, for me. Listen to the intense atmosphere of Oblivious' chorus. Mystical, dark, introspective with all the genius we would expect from Kajiura with three amazing voices, yet not overbearing at all.
For Kajiura Yuki, Kalafina's songs were definitely different than her FictionJunction project, while incorporating our favorite components from her composition styles we may recognize most in her FictionJunction projects.
There's nothing that she, and they, can't do! Except maybe... rap. :P