Maybe I should post mine too ...

Hmm, let's see.

Crysis 2, Killing Floor, Killing Floor 2, Dota 2, Diablo 2 LoD, Half-Life 2 and its episodes, Arma 2 (DayZ mod), GTA V, World of Warcraft, Diablo 3 and had some fun in Left 4 Dead (both of them) back in the day.

Edit - Oh, you changed it to 2015. That would be only Killing Floor 2, Shadow of Mordor and GTA V for me then. I don't think that I've played any other good game released in 2015.

Another edit: Forgot about Rocket League.

Dirt Rally

Mad max
dying light
kholat
windward
Savage land
How to Survive 2
Forward to the Sky
Apotheon
Statues
STASIS

EDIT with games of 2015

Do you mean of the year, or of all time?

So, games we played in 2015 or games of 2015 we played?

Edit:

1. Witcher 3
2. Mad Max
3. Gurumin: A Monstrous Adventure
4. SUNSET

Haven't played too many 2015 games this year.

Games I played in 2015:

1. Witcher 3
2. Mad Max
3. Kingdoms of Amalur: Reckoning
4. Castlevania: Lords of Shadows 2
5. DARK SOULS
6. Of Orcs and Men
7. METAL GEAR RISING: REVENGEANCE
8. Gurumin: A Monstrous Adventure
9. Thief
10. Dead Rising 2

Do they need to be in any order? Assuming you ask about games I played in 2015:

Fallout 2
Batman Arkham series (but not really Knight, something just doesn't click there)
Left 4 Dead 2
Payday 2 pre-Weapon-Rebalance-Patch (current meta makes game stupidity easy)
Killing Floor 2
Borderlands 2
Tropico 4
Civilization
Dying Light
Mortal Kombat

Edit: ok, so you ask about games from 2015, will be a bit harder, but let's try:
Killing Floor 2 (well, Early Access, so not sure if it fits 2015 :P )
Dying Light
Mortal Kombat X
Shadow of Mordor
...
...
Do I have some other games from 2015 I played?
Well, those from 2015 I want to play:
Witcher 3
Warhammer: End Times - Vermintide
Metal Gear Solid
Mad Max
Just Cause 3

The Thalos Principle, Monstrum

1. Witcher 3
2. Fallout 4 (This is closely contested with W3)
3. Batman Arkham Knight (Despite running terrible there's no doubt it's one of my favourite games of all time)
4. Mad Max
5. Metal Gear Solid V: Phantom Pain
6. Dying Light
7. Fifa 16 (No hate)
8. Dark Souls II Scholar of the First Sin
9. Grand Theft Auto V
10. Assassins Creed Syndicate

Please bear in mind I haven't yet played Just Cause 3.

Yay! :)
Love the warhammer franchise (even played the tabletop), so take my submission ^_^

1. CS:go (addicted right now :( )
2. Hearthstone
3. Skyrim
4. To the Moon
5. Stick of Truth
6. Dayz
7. Hitman Absolution
8. Bioshock Infinite
9. WoW
10. Runescape
11. Battle for middle earth

*these are the games I enjoyed the most this year, dont have the money for up to date AAA titles :P

From what I've seen the best game released 2015 was Fallout 4 and Mad Max :)

Witcher 3, ten times

1. Sloth, 11 years, MUD, text based
2. X-com
3. Descent
4. Doom
5. Quake
6. Warcraft 2
7. Counter-strike
8. MU online, 1 year, mmo
9. Wow, 4-5 years, mmo
10. Dota 2, 3 years, moba

2015

1. The Binding of Isaac: Rebirth
2. Ori and the Blind Forest
3. Hyphen

Well, here's mine:
Axiom Verge, Dear Esther, A Story About My Uncle, To The Moon, Super Meat Boy, The Bug Butcher, Tomb Raider..

Bioshock, Biochosk 2, Diablo 3, Sid Meiers V, Sain Row 4, The Witcher 3, Fallout 3, Fallout 4, Arma 2, Far Cry 3

1. Darksiders: Deathinitive Edition
2. Dr. Langeskov, The Tiger, and The Terribly Cursed Emerald: A Whirlwind Heist
3. Relic Hunters Zero
4. Absconding Zatwor
5. Brilliant Bob
6. Burstfire
7. Dungeon Souls
8. GabeN: The Final Decision
9. It came from space, and ate our brains
10. Why So Evil 2: Dystopia

1-Witcher 3
2-Fallout 4
3-GTA 5
4-Mad Max
5-Just Cause 3
6-Dirty Bomb
7-Dirt Rally
8-Ark Survival evolved
9-Vermintide end of times (Beta)
10-Victor Vran.

1. Star Craft --forever 1 !!
2. Company of heros
3. Dark Soul
4. Gran Trismo
5. Call of duty
6. Hero of Strom
7. Final Fantasy seven
   8.Mark of Ninja
8. Darksider
9. Jamestown

GTA V, Witcher 3, Fallout 4, Ori and the Blind Forest, Metal Gear Solid 5: The Phantom Pain, Rocket League, Starcraft II: Legacy of the Void, Tales from the Borderlands, SOMA and Her Story.

Mine would be:

Dirt Rally
GTA V
Mad Max
Mortal Kombat X
Project Cars

Nothing has really got me hooked this year though :|

I just saw your edit for any release year:

Eve Online
Defiance
Dungeon Keeper 2
CS: GO
The Lord of the Rings Online

1) Halo 3
2) Gears of War (Saga)
3) Dark souls 1 and 2
4) Monster Hunter tri Ultimate
5) Mass Effect (Saga)
6) Left 4 Dead 1 and 2
7) Dota 2
8) Borderlands 1 and 2
9) Team Fortress 2
10) Dead Space trilogy

witcher 3
Crusader Kings II
The Talos Principle
War for the Overworld
Tropico 4
Europa Universalis IV
Galactic Civilizations III
Cities: Skyline
Final Fantasy X (PS2)
Unreal Tournament 4

Witcher 3
Dark souls 2
The long dark
Mad max
Max payne 3
Warhammer series
This war of mine
Bioshock infinite
Farcry 4
Deponia

The Witcher 3 Wild Hunt
Rocket League
Metal Gear Solid V: The Phantom Pain
This War of Mine
Final Fantasy VI (currentely playing it)
Tales From The Borderlands
The Binding of Isaac: Rebirth
Borderlands 2

And I can't remember any other at the moment ;P Have a happy new year!

1. Darksiders 1/2
2. Left 4 Dead 2
3. The Binding of Isacc : Rebirth
4. Remember Me
5. Shadow Warrior
6. Call of Jaurez : Gunslinger
7. Portal 2
8. Torchlight 2
9. Call of Duty 4 : Modern Warfare
10. Trine 2