With the recent announcement of Mega Man 10, several great remixes of the theme of the game have been made. This has become one of my fave Mega Man themes EVER.
http://www.youtube.com/watch?v=x7q55uFAk7M
It almost feels like a perfect marriage of the 8-bit Mega Man style music with the 16-bit Mega Man X style music. Yummy!
On a related note, how great was Splash Woman! She had one of the most mesmerizingly beautiful (and still epic) stage themes a water level has ever had.