Oh man Tori no uta is one of my all time faves, even before I even really knew what air was I liked the song, and it has only grown on me more and more as I become more of a AIR fan from watching the anime and reading the VN. I think a big reason why I even watched the anime was cause I liked the op so much and that would eventually lead to reading the vn so you could almost say that the song got me into air! Ever since then I really followed Lia quite a bit too, even the non key music. She has really good English singing too. As far as the other songs, there is natsukage (summer lights(夏影)) that song is super good too. I could name more but I’ll stop there.
As for Kanon’s music I haven’t listen to it much but that will change once I decide to read Kanon which probably won’t be too long from now I do vaguely remember it from the anime, having a very wintery mood to it.