For me it's Justice League Heroes and Hulk UD.
Star Wars: Knights of the Old Republic!
Star Wars Battlefront II
battlefront 2, KOTOR 1 and 2, republic commando, fable, evil dead
KOTR 1 and 2 and I need to find out if Morrowwind (Elder Scrolls 3) plays
well, I might not play Xbox originals on my 360 anymore, but my favorite games on the original Xbox are:
-Halo
-Halo 2
-Oddworld Stranger's Wrath
-Conker
-Jade Empire
-Hulk UD
-Far Cry Instincts
also the Prince of Persia series, Beyond Good & Evil, and KotOR are awesome, but I played those on other systems.
Morrowind (which does play with an update) and Crimson Skies
I played KOTOR a few months ago on the 360. That game still outranks a whole bunch of other games that I've ever played, even on the 360.
FORIAMSPAM!
A sadly overlooked classic platformer by the name of "Voodoo Vince"