Normally, I would say Castlevania: SoTN, since pretty much, it's my #1 game of all-time.
However, I have to go with Bionic Commando: Rearmed.
I've always been a B.C. fan, and when I first heard about B.C.R., I was ecstatic. I've followed it since the first word of it's existence, and doing so paid off. To show my dedication to it, I was actually in FL (live in NJ) when it came out, and I bought a router for $75 (cheapest one down there) just so I could hook up my 360 online, just long enough to download and play B.C.R.
The game did not disappoint. Mixing difficulty, with beautiful HD graphics, with a completely re-done engine, including rag-doll physics. Everything about this game, was just down right, and is so true to the original (which is probably my favorite NES game). I'm not sure about everyone else, but for me, the call-back to the classic, everything they did to exactly replicate the experience of the NES version, all the new elements they added, all the small details (including enemy locations, which match the original), and just everything else combined with fantastic gameplay, and so much replayability, makes this game my favorite XBLA title.