I'm gonna say the shmups genre. I'm surprised of all those shmups thats available on the 360 and a couple of gems are even region free. Add also those shmups available on Live. --- In theory there is no difference between theory and practice - but in practice, there is.
Bladestorm: The Hundred Years' War - I loved this game. I just wish it had online co-op. Still fantastic. Rayman Origins - Seems pretty unknown except perhaps on these forums. Best platformer I've played in years and one of the best game all around I've played as well.
CHROMEHOUNDS is a good one too. Sucks they took down the servers.
TC, I'm not sure if you understand what "hidden gems" is supposed to mean. You named a bunch of triple A titles.
But my list...
Singularity Vanquish Binary Domain Brothers In Arms: Hell's Highway Spider-Man: Web of Shadows --- "Nothing gets her hot like seeing you do 5G barrel rolls until you nearly black out." -Elitas Travel Official Walking Champion, GTAO Xbox 360 Board