White Destiny from Pretear. Morning Grace from Princess Tutu and Aiyaa Four Thousand Years were my two top candidates, but since they're taken, I'll go with my current favorite song. I mean, just listen to it! This song is the reason I adore Yoko Ishida(she sings it, btw)!<3333 Pretear is the only anime I've ever watched where I didn't start skipping the intro after the first few episodes.
Another anime song I really love is Brothers from FullMetal Alchemist. Vic Mignogna's English version is just so gorgeous, I can't even begin to explain it! It's even sadder if you've seen the anime and you know what Ed's been through... T~T
Brothers:
link
White Destiny: