Zelda u, of course :D

Metal Gear Solid V: The Phantom Pain. I'd love to have the time for this game only and beast at it.

Definitely Xenoblade. I love Zelda, but Zelda can wait for Xenoblade.

Halo 5 would be my #1 hands down

I choose Metal Witcher Solid 1886

Exactly, means I get to play the game before everyone else =P

Xenoblade Chronicles X, any other game wouldn't hold nearly enough content to last 2 months let alone an entire year.

touche, lol.

hearts of iron 4

If I had only one game to play I would pick an MMO or a sim-racer. Enough to do in either of those. I also do like longer Single-Player-RPGs (SPRPG) but I usually need to play other games between SPRPG sessions, so a SPRPG isn't my first pick if it's the only game to play.

Not sure which game I would actually choose. Most likely WoW, FF XIV, Project Cars or GT6 but I assume that you only meant games releasing in 2015. From the announced ones there isn't actually a game that I would want to choose. I'm hoping that Gran Turismo 7 get's announced and released next year, but 2016 is probably more likely for that to happen.
Star Citizen is another game I'm waiting for, but I'm kinda expecting that a full release will be in 2016. I already have backed the game, but I'm not sure yet if I'm really interested in all this persistent-universe kind of stuff. I hate microtransactions that give even the slightest advantage in multiplayer games, so I will most likely focus on the single player part of the game. And then there might not be enough stuff to do in a whole year.