Libera me from hell it's the greatest without a doubt.
Others i like:
Sanctuary from Earth girl arjuna BGM
http://www.youtube.com/watch?v=DYR_LNgMUmc
Rewrite from FMA OP 4
http://www.youtube.com/watch?v=laWzZlJAR1k
Shiki no uta - Samurai Champloo ED
http://www.youtube.com/watch?v=8REhEEnEheY
Shu Aka - Cyber team in akihabara BGM
http://www.youtube.com/watch?v=m4zZUJMzJP4
Will - Houshin Engi (Soul Hunter) OP
http://www.youtube.com/watch?v=zPGx0p0Ndfo
Konna ni chikaku de - Nodame Cantabile first season ED 1
http://www.youtube.com/watch?v=i-bHQKpOWmk
And that's all.