Personal Killer Apps.

[Glass Joe]

NES: The obvious ones plus Star Tropics and Milon's Secret castle
Genesis:Landstalker,Contra HC (I could have just had these 2 games and it would have been worth it.),Ghouls and Ghosts-the game I bought the Genny for.
SNES:Super Metroid,SFII,Super GnG
Turboduo/Graphxracula X,Legendary Axe,Neutopia
PS1:Alundra,Tomb Raider 1,RE1,Castlevania SoTN
N64:Mario 64,Zelda OoT,Excitebike 64(Still my favorite racing game ever)
PS2:Ico,Soul Reaver 2,MGS3,Devil May Cry
GC:Sphinx,ZeldaWW,Dave Mirra 2
XBOX:Splinter Cell PT,Ghost Recon,PoP2 WW
GBA:WarioLand 4,Castlevania CotM,Ninja 5-0
Obviously there are tons more that I can't even think of. I couldn't think of ANYTHING for the Saturn or the DC that I would consider "Killer app" material.

[Clash!]

Maybe if people would put comments with their lists rather than just typing a list.

SNES - Super Mario World/Super Mario Kart/Contra III: Me and my brother rented the SNES like 6 times with these 3 games prior to the Christmas that we got it, with those 3 games.

Genesis - Gunstar Heroes: Never bought this system till I was like a senior high school. I was never a Sega fan, but all of the GameFans praise of its "hardcore" library made me buy one. Gunstar was the first game on my list.

PlayStation One - Battle Arena Toshenden 2/Need For Speed: Yes BAT2, not BAT1. My friend had got a PS1 well before I did and we were blown away by BAT, and then when I got mine the same week 2 came out and was beyond excited for it. Now of course when I play them it plays like crap, but 3D was much more impressive back then. And I got NFS because of the demo I played 8,000 times at my friends house.

Second PSOne - Street Fighter Alpha 3: After selling my PSone to a buddy for an N64, I eventually rebought it when Street Fighter Alpha 3 came out.

Nintendo 64 - Mario 64: I would play this at Wal-Mart for a couple hours a week while saving up for the system and the game. Also sold my PSone and its games to a friend to get money for it. Mario 64 was worth it. Funny note, it was Christmas time when I got my 64, and there was such a demand for Mario 64 that Funcoland was selling it for $99, scoffed at that and walked two blocks to Target at got it for the standard $60.

Dreamcast - Sonic/PowerStone: Had to have these games, they looked so freaking amazing. Power Stone is amazing, Sonic Adv.s horrendous hub kills the game for me. Levels for the most part are fun, but who the hell wants to replay them as Big the Cat or Amy?

PS2 - No Game in Particular: Got it as a gift, but wanted it for what was obviously going to come. First game I bought for it was Street Fighter EX 3.

GameCube - Super Smash Bros. 2: Its a fantasticly fun game.

Xbox - Gunvalkyrie: Was not anticipating Halo from what I had seen of it, so nothing made me want it on Day One. The demo of Gunvalk however got me really interested and picked up an Xbox with Gunvalk when that game came out.

[Videodrone]

Glad to see the Powerstone love.

[NeoZeedeater]

Almost all the games that convinved me to buy systems were launch or early titles. Instead of making a giant list I'll list a few underrated system sellers for me.

Plus, everyone knows I can't resist posting screenshots of games.

Commodore 64 - Aztec Challenge
This was definitely one of those transition era games in between the "play for score" era and the "scroll/boss" era. It was an obstacle course with various levels: side view, overhead, front view, isometric.


Sega Master System - Safari Hunt
My jaw dropped when I first saw this and I knew I needed one of those newfangled Sega contraptions. It was like a cartoon come to life and managed to surpass Duck Hunt in looks, gameplay and variety which was impressive since Duck Hunt was already one of my favorite games.


TurboGrafx-16 - Alien Crush
A lot of early TG16 stuff impressed me. This stood out by being the best and most unique video pinball game ever made. Nothing like this had been attempted before.


Virtual Boy - Mario's Tennis
I gave it try on display at Blockbuster and I was sucked in. It was its own Nintendo microcosm. Good times.


Playstation - Warhawk
I got my PS1 for Christmas 1995 and this was the first game I bought a day later. There were a lot of brilliant new 3d games that season(on Saturn too). Warhawk was one of the cooler ones.

[Wilykat]

Dreamcast: Sonic Adventure and Street Fighter III
PSX: none, I don't know why I bought one on launch. Toshinden?
N64: Super Mario 64, like everyone.
SNES: everything
Genesis: Sonic 1
PS2: Metal Gear Solid 2
GameCube: Super Smash Bros. Melee.
XBox: modding, emulation box

I'll avoid the more obvious ones like "NES: it came with SMB" and just end it.

[Cowdisease]

NES - After seeing Super Mario Bros in action, I had to have one. Playing Super Mario Bros for the first time brought me immense joy, and it planted the seed of my video game hobby.

Genesis - As a rabid Nintendo fanboy, I hated the Genesis, especially with their "Nintendon't" commercials. I quickly changed my tune when Sonic the Hedgehog and Streets of Rage came out and asked for the system and games for my birthday.

Super NES - My Nintendo fanboyism forced me to buy this system regardless of what games were coming out. Thankfully, Super Mario World was packed-in with the system to justify the purchase.

Game Boy - Again, Nintendo fanboyism prompted me to get the first portable Nintendo system. Tetris was a great pack-in though.

Turbo Graphx-16 - This was more of a want for the system than a "I have to play that game!" Bonk's Adventure helped justify the purchase. Looking back, I'm surprised I didn't buy Alien Crush for the system.

32X - I didn't have a good PC and I wanted to play Doom. I regret the decision considering that the only other games I played on it were Knuckles Chaotix and Virtua Racing.

PSX - I was hesitant to get the system as first, but after playing Street Fighter Alpha at a friend's house, I had to get it.

N64 - I surprisingly passed over the system even with the release of Mario 64. My justifications were that it seemed like it wasn't going to get a lot of games, and my friend who lived down the hall had one, so if I wanted to play it, I could just go to his house.

Dreamcast - The release of Marvel vs. Capcom 2 prompted me to buy a Dreamcast in 2000 with Sonic Adventure and KOF Dream Match. The online plans and Quake 3 Arena (again, I had a sucky PC) were the other determining factor to get the system, even though they weren't out yet.

Gamecube - I preferred the Gamecube launch games over Xbox's, with Super Monkey Ball being the best reason. Smash Bros. Melee, which wasn't out yet, also convinced me that I wanted the system.

PS2 - It was GTA: Vice City that prompted me to get the system. I borrowed a PS2 and GTA 3 before, and I really liked GTA 3, so I looked forward to playing the sequel (which later disappointed me). Toys 'R Us's "Buy 2 Get 1 Free" sale was the other reason to finally pick up the PS2 (I don't remember all the games I bought, but Contra: Shattered Soldier was one of them).

Xbox - Halo 2 was the reason that I finally got the Xbox. It was also the reason that prompted me to get broadband and Xbox Live.

[Glass Joe]

I'll avoid the more obvious ones like "NES: it came with SMB" and just end it.

Mine didn't. I got Rob the Robot and Gyromite with mine. Maybe the gun and duckhunt too but I really don't remember.

[Psx]

Super NES: Mario...Remember playing it at my friends house and thinking WOW I NEED THIS!
Playstation:I don't think there was any one game, but damn do I remember when I couldn't afford a game and played that demo disc to death.
N64:Mario 64, also my first experience with an import game as I rented the system from some store way before the US release.
Gamecube:Mario again
All the other systems I own I really didn't buy for any one game

[Wilykat]

Playstation:I don't think there was any one game, but damn do I remember when I couldn't afford a game and played that demo disc to death.

I did that for about a month as well. I sold nearly all my videogames, including relatively new SNES games like Chrono Trigger, just to get one on launch. What really got my interest in the system going was the screenshots of games I was seeing in GameFan. Stuff like Gunners Heaven, Arc the Lad, and Toshinden.

After buying the system, I was completely broke, so I played both the preorder demo disk (the one with the dinosaur demo) and the one that came with the system for about a solid month. Then I got Toshinden.

Then I decided to start saving up to re-buy an SNES and all my games. I didn't like the PSX for the first year, save Jumping Flash.

[Tojo]

I dropped $400 for Tekken Tag Tournament. That was the only game I owned for about 8 months and it was worth every penny.