Topic: If you could only have one game for every system that you own...

GBA: Nes classics: Donkey Kong
GC: Super Smash bros. Melee
PS2: Sitting Ducks
DS: Professor Layton and the lost future
Wii: Super Mario all stars 25th aniversary edition
3DS: Professor Layton and the Azran legacy
Wii U: Dr Luigi

GB: Super Mario Land 2
GBA: Mario & Luigi Superstar Saga
DS: Metroid Prime Hunters
2DS: Super Smash Brothers 3DS
N64: Super Mario 64
GCN: Super Mario Sunshine
Wii: Xenoblade Chronicles
Wii U: Mario Kart 8
PS2: Sly Cooper 2
Xbox 360: Skyrim

Let's see, I have a PC, N64, PS2, Wii, 3DS, and PS3. I'd have to get... Nope, too many great games to pick from. I give up.

NES: Mario 3
N64: F-Zero X
GBC: Legend of Zelda: Link's Awakening DX
GBA: Pokemon Emerald
PS2: God of War
Gamecube: Metroid Prime 2
DS: Metroid Prime: Hunters
PS3: Mass Effect 2
Wii: Legend of Zelda: Skyward Sword
3DS: Pokemon Y
Wii U: Legend of Zelda: Wind Waker HD

N64: F-Zero X
GBC: Pokémon Yellow
GBA: Fire Emblem
GCN: F-Zero GX
DS: Mario Kart DS
Wii: Metroid Prime Trilogy
3DS: Kid Icarus Uprising
Wii U: Mario Kart 8
PS1: Tomb Raider The Last Revelation
PS2: Amplitude
PS3: Tomb Raider (2013)
Xbox 360: Rock Band 3

SNES- Super Star Wars (Most of my SNES games are downloaded on my Wii, I don't have many for the system itself)
N64- Star Wars Episode One Racer (Don't have many games for this either)
Gamecube- Animal Crossing
Wii- Xenoblade Chronicles
Wii U- Mario Kart 8
Gameboy Color- Pokemon Silver
Gameboy Advance- Pokemon Emerald
DS- The World Ends With You
3DS- Super Smash Bros
PS1- Crash Bandicoot Warped
PS2- Sly Cooper 2
PS3- Kingdom Hearts 1.5
PSP- Crisis Core
PC- Half Life 2

Genesis—Pitfall: The Mayan Adventure
GB Pocket—Super Mario Land 2
N64—Donkey Kong 64
GBC—Super Mario Bros. Deluxe
GBA—Mario Kart: Super Circuit
GCN—Sonic Mega Collection
DS—Planet Puzzle League
Wii—Super Smash Bros. Brawl
3DS—Donkey Kong '94
Wii U—Sonic Lost World
PS1—Crash Team Racing
PS2—Super Bust-a-Move
PS3—Sonic Unleashed
Mac—Earthworm Jim 2
…iOS—Pac-Man (because I haven't bought Ms. Pac-Man yet)

NES: Super Mario Bros. 3
SNES: Super Bomberman 2
N64: Star Fox 64
GCN: Super Smash Bros. Melee
Wii: Xenoblade Chronicles
GBC: Pokémon Red & Blue
GBA: Metroid Fusion
DS: Dragon Quest IV
3DS: Fire Emblem: Awakening
Genesis: Strider
Dreamcast: Capcom vs. SNK 2
Neo-Geo: Metal Slug 2
PSX: Gundam Battle Assault 2
PS2: Devil May Cry 3
PS3: Valkyria Chronicles
PSP Ys Seven
PS Vita: Ys: Memories of Celceta
Xbox: Halo: Combat Evolved
Xbox 360: Lost Odyssey
PC: Team Fortress 2

This is hard...

NES: The Adventures of Lolo & Lala 3
GameBoy/Colour: Tetris
GameBoy Advance: The Legend of Spyro: The Eternal Night
Nintendo 64: The Legend of Zelda: Ocarina of Time
GameCube: Pikmin 2
DS: Pokémon SoulSilver
Wii: Super Mario Galaxy 2
3DS: Super Smash Brothers
Wii U: Mario Kart 8
PlayStation: Spyro 2: Gateway To Glimmer
PlayStation 2: The Legend of Spyro: A New Beginning
PlayStation 3: Skylanders: SWAP Force
PlayStation Portable: [PSX Classics] Crash Bandicoot: Warped
XBox 360: Viva Piñata: Trouble in Paradise
PC: Minecraft
iOS: Jetpack Joyride

I tried to keep a good variety with my selections, trying to pick the games with the best game design/longevity of their series that I could. That didn't always mean picking my favourite from each series. I also only chose games I've actually played, which gave me only a couple choices for some consoles.
_{Ocarina of Time- I've played the 3DS remake, but not the original. Also with Pikmin 2 I've only played New Play Control edition.}

About that PSP game, the only other choices I had were Crash of The Titans/Mind Over Mutant and the Monster Hunter games, but I think I'd hardly play either. I'd play an old Crash PSX game over either, any day.
If I really have to pick a "proper" PSP game, I'd go with Mind Over Mutant. Crash Bandicoot: Warped would become my PSX pick, then I'd choose Spyro: A Hero's Tail for the PS2. I just can't stand grinding, and Monster Hunter is full of it. SoulSilver has grinding too, but way more nostalgia protecting it

Provided SSBU is as good as the 3DS version (which it will be), I'd replace Mario Kart 8 with it and choose Mario Kart 7 as my 3DS game instead. I prefer MK7 to MK8, so... I'd be happy with that.

It's a real shame I couldn't fit an Animal Crossing game in there. I ultimately decided I couldn't keep playing the older games and they just feel outdated after New Leaf (which I don't play much). Besides, I get bored of them if I can't talk about them with people who also play.
The original was actually going to be my GameCube pick, before I remembered Pikmin. Switched in a heartbeat.

NES pick was a tossup between Balloon Fight and The Adventures of Lolo & Lala 3. I picked L&L3 purely because I think I'd get more out of it.
PS3 pick was a tossup between Skylanders: Giants and SWAP Force, but again, picked the latter just because it has more longevity.

GameCube: Super Mario Sunshine. That's all I need. A system that lasts an eternity, and a game I can play infinitely over again.

Atari 2600: Warlords
NES: Super Mario Bros. 3
Genesis: Sonic 2
Gameboy: Donkey Kong
N64: Ocarina of Time
PC: Starcraft
Gamecube: Smash Brothers
Playstation 2: Star Wars Battlefront II
GBA: Fire Emblem: Sacred Stones
Wii: Twilight Princess
DS: Advance Wars: Dual Strike
3DS: Fire Emblem: Awakening
iOS: Walking Dead