I love any version of Hatsune Miku's Last Night, Good Night or Love Is War, especially some of the covers by the NicoNico folk such as Choucho and Usa.
I'm also exceedingly fond of K-ON! music such as No, Thank You! and Don't Say 'Lazy'.
My favourite, though, is Lia's My Soul, Your Beats! from Angel Beats!. Epic song. Actually, most of the music in AB! is epic, especially Karuta's version of Ichiban no Takaramono.