PC - Free-ish - Demo of Armadillo Run/Mungyodance 2
Bought - CounterStrike, but Morrowind come in a close second.
Playstation 1 - Either Legend of Dragoon for the sentimental value, or the 3rd Spyro game for the Awesome value.
PS2 - Exclusive - Shadow of the Colossus
Xbawx 360 - Dead Rising. You would expect me to say Halo 3, wouldn't you?
Wii - What little of the new Metroid I've seen/played, it was pretty awesome.
Multiplatform - Guitar Hero III. HEIR KOMMT ALEX.