Defining "system seller" as "first game I bought for the system":
N64 - Super Mario 64
GameCube - Super Smash Bros. Melee
PS2 - Final Fantasy X
DS - Super Mario 64 DS, Pokemon Diamond Version
Wii - The Legend of Zelda: Twilight Princess
3DS - The Legend of Zelda: Ocarina of Time 3D
Wii U - Mario Chase (Nintendo Land)
PS3 - Demon's Souls
Other games that definitely helped justify these purchases:
N64 - The Legend of Zelda: Ocarina of Time, Majora's Mask, Super Smash Bros., Banjo-Kazooie, Jet Force Gemini, Banjo-Tooie, Perfect Dark
GameCube - Mario Kart: Double Dash!!, Metroid Prime, Metroid Prime 2
PS2 - Final Fantasy XII, Kingdom Hearts, Kingdom Hearts II, Katamari Damacy (never wound up buying much for PS2)
DS - New Super Mario Bros., Pokemon SoulSilver Version
Wii - Wii Sports, Super Mario Galaxy, Super Mario Galaxy 2, Okami, Metroid Prime Trilogy, New Super Mario Bros. Wii, Donkey Kong Country Returns, The Legend of Zelda: Skyward Sword, Xenoblade Chronicles, The Last Story, Rayman Origins, the entire Wii Virtual Console library
3DS - Super Mario 3D Land, Mario Kart 7, The Legend of Zelda: A Link Between Worlds, Kid Icarus Uprising, Fire Emblem: Awakening
Wii U - Pikmin 3, Super Mario 3D World, Donkey Kong Country Tropical Freeze, Mario Kart 8
PS3 - Dark Souls, Kingdom Hearts I.5 HD Remix, Final Fantasy IX (just bought my PS3 in June!)