Most of the soundtrack to Aquarion (show=meh. soundtrack=Yoko Kanno)
I still like Hemisphere (RahXephon) though it might be for more sentimental reasons.
Sakura Kiss from Ouran High School Host Club is really catchy.
God Knows, the first song they play in episode 12 of The Melancholy of Haruhi Suzumiya is pretty awesome.
Taiyou no Mannaka He, the 3rd opening for Eureka 7 rawks.
Hanabi, the current ending song to Bleach, was unique and got me into the group that sings it: Ikimono Gakari. They have a really good sound, the harmonica is a nice touch.

Those are the songs that stand out immediately, but as you see it's hard to pick just one :P