Wii U: Xenoblade X
The other two systems I wouldn't need a game. This will last me a year.

Do I have to choose games releasing in 2015, or can I pick games from their complete libraries 'til now? Assuming the former, I'd pick:

Wii U : Xenoblade X
XOne: Forza 6, if it releases this year; else: FF XV if that one releases this year; else: Project Cars
PS4: GT 7 if it releases this year; else: FF XV if it releases this year; else: Project Cars

But there probably are other games releasing in 2015, that I'm just not thinking about at this moment or that haven't been announced yet, that I could prefer to the games I listed.

What?  No way.  That's crazy.  What does it mean?

X in this case means... ahem.... X.

Never gave 2 thoughts about it until now.  What the hell is the X for? I'd say one of the 3:

1) Something related with the game we don't know yet

2) Since it isn't a direct sequel to the 1st, they didn't wanted to call it XC2 like Square does with the Final Fantasy franchise, but also at the same time they wanted to let people know it is a spiritual sequel so hence the... X.

3) They just think it is cool

Of course this might change with reviews and more info on the games but for me:

Wii U: Devil's Third
XOne: Quantum Break
PS4: Final Fantasy XV

But tough question OP, tough question.

Wii U - Zelda U
PS4 - Uncharted 4
Xbox One - Mortal Kombat X
3DS - Fire Emblem
PC - Arkham Knight
Vita - A Japanese RPG

Are we assuming FFXV is coming out this year? Because i doubt it.

i am gonna say:

Wii U: Xenoblade X
PS4: Witcher 3
X1: Err... nothing.

Good to hear! I was hoping this would prove something for people to challenge themselves with.

X1: Halo 5
PS4: Bloodborne
Wii U: Xenoblade
PC: Witcher 3

PS4: Uncharted 4: A Thief's End
Xbox One: Rise of the Tomb Raider
WiiU: The Legend of Zelda