All of this XBox 360 hype has made me reminisce a bit about what were the first "killer apps" for all of the systems. Even unsuccessful systems have them. Some of my picks:
Atari - Truly not knowledgeable enough to make a real judgment. I would personally say Combat. Great pack-in. Pitfall and Asteroids did it for me too. Woulda/coulda/shoulda been Pac Man.
Colecovision - Donkey Kong.
Intellivision - ??????
NES - Super Mario Bros. Is there any question? It only outpaces Zelda due to its earlier release.
SMS - Tough call. I want to say Phantasy Star, but Space Harrier really blew my mind at the time.
Gameboy - Tetris. Still my favorite version.
Atari Lynx - Probably Blue Lightning.
Genesis - I think the initial arcade based linup rocked, but the answer is Sonic the Hedgehog. Made them a true competitor.
Sega CD - Sonic CD and Lunar. Final Fight CD was underappreciated at the time, as was Silpheed.
TurboGrafx - Best early game is probably Legendary Axe. All of them are sleepers though for different reasons. Hell, China Warrior was cool then, and I still dig Keith Courage.
TurboGrafx CD - Would have been Drac X, but Ys I & II was the first CD game to generate real buzz.
SNES - Super Mario World. Runner Up: F-Zero/Pilotwings. Rarely have three launch titles been so mindblowing.
Game Gear - Probably Columns.
32X - Virtua Racing Deluxe was the only reason to buy the 32X unless you liked being disappointed with Star Wars Arcade.
Sony PSX - Ridge Racer hands down. Still amazing.
Sega Saturn - Virtua Fighter 2/Sega Rally. Made up for the disasters that were VF and Daytona, unfortunately the damage was already done.
N64 - Super Mario 64. Had to have it. Zelda came too far down the road, though the buzz certainly sold systems.
Dreamcast - Soul Calibur.
GBC - Hmmm...
GBA - I would go with F-Zero again. Castlevania got killed by invisibility and SMB Advance was just SMB2 in nicer clothes. There wasn't too much that really justified the jump at the time outside of backwards compatibility and a larger screen. I am including the SP with this.
PS2 - Didn't get a true killer app until MGS2 and GT3 came out.
XBox - Halo.
GameCube - Uhhh... oh yeah, Metroid Prime.
DS - Super Mario 64 most likely wins again, but not in terms of being an true application of the the technology. You could go with Feel the Magic, but Trauma Center: Under the Knife is truly the best use of the touch screen/dual screen setup so far. Too bad no one knows.
PSP - Lumines. Ridge Racer is awesome too.
XBox 360 - Not yet.
I know I am missing systems, but you get the idea.