A theory on videogame system sales is that a system needs killer apps in order to get sufficient sales to be a success.  The "killer app" is one that people MUST own.  To this end, I figured I would look back over the history, and discuss what seems to be these, asking others to chime in and correct, and add their own.  I go a bit loose here, in also using aspects of videogames. 

Let me start with Atari 2600 days and go to current:

* Atari 2600: Arcade games.  These included Space Invaders, Pong, and Asteroids. 

* NES: Super Mario Bros.  2D scroll games were the must-have these days.

* Genesis: Sports titles, Mortal Kombat, Sonic.  The Genesis made a name for itself with Sports titles. 

* SNES: 2D side scrollers (complete with colorful graphics), JRPGs, Street Fighter 2.  The SNES grabbed these and pushed to be sales.  I believe the SNES marked the last hurrah for the relevance of arcade games relating to home sales.  There was early hints at 3D, but that was just Star Fox.  Donkey Kong Country also would be as relevant as Star Fox I believe.

* Playstation: JRPGs with the use of FMV, polygons: Tomb Raiders, Tekken.  This was the breakthrough to the use of Polygons and 3D.  Add a little title called "Gran Turismo" to the mix to (yes, little is sarcasm there).  I am tempted to say Metal Gear Solid, but I believe it took PS2 to establish it on the map on the movie experience game.

* Playstation 2: DVD, Grand Theft Auto.  More video, and also sandbox games.  The PS2 and XBox (see below) also signaled the end to sprites and 2D being significant to videogames.

* XBox: Halo.  Halo put the XBox on the map, along with the genre of FPS, that suddenly became mainstream in sales, and a phenomenon.

* Wii: Wii Sports and Waggle.  Motion controls came on the map with this, blindsiding everyone.

I don't think they have to be killer apps, they just have to stand above and aside from multi-plat titles. Halo 3/Gears of War are both system sellers whilst Killzone 2 and RFOM 1/2 are not because the former are both unique and/or better than the multi-plat titles at the time. Gears of War stands aside from Call of Duty, but Killzone 2/RFOM 1/2 can't get out from under its shadow.

You missed a couple of insignificant systems called N64 and Gamecube.

"XBox: Halo. Halo put the XBox on the map, along with the genre of FPS, that suddenly became mainstream in sales, and a phenomenon."

I don't get this. Which become phenomenon? XBOX or FPS genre?
If first - then XBOX never become phenomenon.
If second - FPS become phenomenon many many years ago before Halo.

I would of left the XBox off also, but it had Halo and a FPS.  Nintendo managed to make the N64 and Gamecube profitable, but they hardly fit into being market leaders, which is why they got left off.  I could of did the same by listening the rivals to the NES and Atari 2600 also.

Halo put CONSOLE versions of first-person shooters on the map.  This is why, even now, it is look at as the king of console shooters.  I was looking at consoles here.  Similar to how RTS has been around for years, but hasn't broken through to the consoles yet. 

I believe that N64 with Super Mario 64 put the 3D platformer on the map.

N64 deserves a mention for SM64 for.. being THE 3D platformer among 3D platformers.

GC also because MP revolutionized First Person Adventure games and to a lesser extent First Person Shooters.

And having the greatest game of all-time, Ocarina of Time and the best FPS until Halo, Goldeneye.

How can we forget Super Smash Brothers Melee?

While all 3 of those games are fantastic and revolutionary in several regards, I believe MP and SM64 were more revolutionary.