PS4: Bloodborne
X1: Sunset Overdrive
WiiU: Xenoblade (although if SMTxFE comes this year, then that)
3DS: Fire Emblem
Vita: no idea

PS4 - Bloodborne
Wii U - Xenoblade Chronicles X
Xbox One - Final Fantasy XV
Vita - God Eater 2: Rage Burst
3ds - Shin Megami Tensei: Devil Survivor 2: Break Record

So far:

PC - Pillars of Eternity (toughest competition here, at least for me)
PS4 - Rime (unless Deep Down turns out to be exceptional)
Wii U - Zelda
XOne - Fable Legends (since Inside is most likely timed)

Other games, like No Man's Sky, that should be up there, are either multiplats or timed exclusives.

WiiU: X
Xbox One: By now Quantum Break but let's see what they have under the hood for E3
PS4: Uncharted 4

Wii U: The Legend of Zelda, as much as I'm loving Splatoon, gotta have that open world and dungeon solving xP

I don't have the other 2 consoles so I don't really care for them, so I'm listing a PC game instead ;)

PC: A Hat in Time, if that even comes out this year.

Only one? That's very easy.

EDIT: PS3: Persona 5
PS4: Bloodborne
Wii U: Zelda U
3DS: Monster Hunter 4 Ultimate

Wii U: Splatoon
PS4: Mortal Kombat X
X1: Halo 5

Not necessarily my favorite games but the ones that I feel would have more lasting appeal because of the multiplayer. Also, only included games I'm sure will be out in 2015

Wii U - Dk Tropical Freeze (cause it would take ages to 200% it)
PS 4 - Street Fighter maybe? Don't know what exclusives they have. If not exclusive then Fifa as it would last forever
Xbox - Sunset overdrive (only game I know of :-p )

Xbox One:Halo 5
PS4: Uncharted
Wii U:Star Fox

PS4: Final Fantasy XV/Persona 5 in case FFXV doesn't come out this year.
Wii U: Xenoblade Chronicles X
XB1: Halo 5