It used to be Castlevania: Symphony of the Night, but I haven't procured my own copy since my roommate moved out many years ago and I refuse to buy it off XBLA.
Other than that, I'll usually gravitate towards shooters between bouts of longer games. Castle of Shikigami, Gradius V, R-Type Final, Deathsmiles, Gunbird 2, Mars Matrix, etc.