The 2015 Game of the Year is The Witcher 3.
The runner up is Bloodborne.
The master thread for all of these contests can be found here.

We are now entering the second half of the decade in 2015.  Generation 8 is fully underway with the PS4 and XBox 1 having significantly fewer cross generation titles than the year before, and the Wii U would have several notable titles as well.  So, without further ado, here is the list of games for 2015:

Undertale (PC/Mac)
Toby Fox first created a few ROM hacks of the RPG Earthbound, before he sat down and created his first original game, Undertale.  The game features story, characters and music of the quality of an exceptional JRPG, but all of this was done by one (American) person.  Undertale scored a 92 on Metacritic, and it has sold over 5.9 million copies just on Steam alone.

Splatoon (Wii U)
This new IP from Nintendo would prove to be especially popular in Japan.  Splatoon would sell over 4.9 million copies on the Wii U.

Super Mario Maker (Wii U)
Nintendo gave players the ability to make and play levels for the most iconic series of video games ever.  Super Mario Maker has sold over 7.8 million copies total on the Wii U and 3DS.

Bloodborne (PS4)
This PS4 exclusive would be a more action-oriented version of a Souls game.  Bloodborne scored a 92 on Metacritic and sold over 2 million copies simply by the end of 2015.

Fallout 4 (PS4/XB1/PC)
Bethesda releases the next entry in their post-apocalyptic series.  Fallout 4 would sell over 25 million copies.

Metal Gear Solid V: The Phantom Pain (PS4/XB1/PS3/XBox 360/PC)
Metal Gear goes open world in this final title which would be directed by series creator, Hideo Kojima.  The Phantom Pain would score a 91-95 on Metacritic and has sold over 9.6 million copies total.

Ori and the Blind Forest (XB1/PC)
While the XBox 1 had fewer exclusives than the 360, it would produce some very interesting new indie titles.  The big one for 2015 would be this visually stunning Metroidvania which would sell over 2 million copies.

Rocket League (PS4, PC)
Use rocket cars to play the world’s most popular sport.  Rocket League would sell over 40 million copies across all platforms.

The Witcher 3: Wild Hunt (PS4/XB1/PC)
CD Project Red, the developers behind the “Good Old Games” website, would strike gold when they developed this third title in the Witcher series.  The Witcher 3: Wild Hunt scored a 91-93 on Metacritic.  It has sold over 50 million copies to date and is still going strong.

Other (please specify)
If you think another game deserves to be "Game of the Year" other than one of the 9 listed above, then please vote “Other” and say your game in the comments.  If the "Other" category takes at least 2nd place and no other game gets at least 40% of the vote, then  there will be a runoff vote including the most mentioned game(s) from the comments.

Voting for 2015 Game of the Year will end on Wednesday, Dec 13 at 11:59 PM, EST.

Here are some other notable games from 2015:

Blazblue: Central Fiction (Arcade)
Tekken 7 (Arcade)
Arena of Valor (Mobile)
Dragon Ball Z: Dokkan Battle (Mobile)
Fantasy Westward Journey (Mobile)
Fate/Grand Order (Mobile)
Cities: Skylines (PC/Mac/Linux)
Crypt of the Necrodancer (PC/Mac/Linux)
Downwell (PC/Mobile)
Galactic Civilizations III (PC)
Heroes of the Storm (PC/Mac)
Keep Talking and Nobody Explodes (PC/Mac/Gear VR)
Pillars of Eternity (PC/Mac/Linux)
Total War: Attila (PC/Mac/Linux)
Animal Crossing: Happy Home Designer (3DS)
Fire Emblem Fates: Birthright/Conquest (3DS)
Loz: Triforce Heroes (3DS)
Mario and Luigi: Paper Jam (3DS)
Monster Hunter Generations (3DS)
Rhythm Heaven Megamix (3DS)
Steamworld Heist (3DS)
Digimon Story: Cyber Sleuth (Vita)
Mario Party 10 (Wii U)
Xenoblade Chronicles X (Wii U)
Yoshi's Wooly World (Wii U)
Assassin's Creed: Syndicate (PS4/XB1/PC)
Axiom Verge (PS4/PC/Mac/Linux)
Call of Duty: Black Ops III (PS4/XB1/PS3/XBox 360/PC)
Disgaea 5 (PS4)
Forza Motorsport 6 (XB1)
Halo 5: Guardians (XB1)
N++ (PS4)
Rise of the Tomb Raider (XB1/XBox 360)
Star Wars: Battlefront (PS4/XB1/PC)
Yakuza 0 (PS4/PS3)

SOMA

No contest here for me, Bloodborne is the best game of the whole generation.

The WiiU had 2 games that got significant playtime from me. Smash and Mario Maker. Both are games that did not get ports on switch but instead new installments very similar to the game on WiiU. Smash Ultimate is more and better without really missing anything from the WiiU game. Mario Maker 2 is also more and better, however it does miss a few things. The WiiU gamepad worked better for the purpose of creating levels than the switch and the amibos customs made for more creations of themed maps based on the customs.
Mario maker get my vote for this year. It was the only game I felt benefitted from WiiUs console design. Even if most people are not good level creators, the amount of them with golden nuggets here and there make it a game one can play for hours and hours.

I do not know if I was out of touch from gaming during this time period. But I do not even know what most games released are.

I am once again voting ”Other” - Life Is Strange

Next year one of the titans of the industry launched, can’t wait for 2016. 

Bloodborne was the first soulslike I played and because of it, I can't get into many others. The way it played, the speed, the trick weapons, the animations. All amazing.

This said, what a year.

The Witcher 3 by a mile, one of the greatest of all time. Undertale is also an amazing game that unfortunately got fairly overlooked at the time, probably made it onto more game of the decade than game of the year lists.

Top 5 for the year:

1. The Witcher 3: Wild Hunt
2. Undertale
3. The Beginner's Guide
4. MGSV: The Phantom Pain
5. Batman: Arkham Knight

Ori is a close number 6.

2015 is the year of Bloodborne, not even Witcher 3 - good as it is - can take that crown away from it. Bloodborne is just THAT game.
Great year tho.