What are some great single player games?

Mass Effect #1

Brothers: A Tale of Two Sons, Borderlands 2 (not need co-op)

This thread is gonna explode!

• Mass Effect (As you noted)

• Dragon Age

• Star Wars Knights of the Old Republic

• Baldurs Gate

• Planscape Torment

• Deus Ex (Also in a Humble now)

• The Witcher (Both games)

Witcher 1&2, Gothic 1&2, Fallout 2 and Fallout New Vegas

Grim Dawn, Deus Ex series and the new Divinity : Original Sin is awesome if you like old school games like baldurs gate etc.

Portal

Bastion

Fallout. all of them, the old ones and the new ones.

Besides the ones already mentioned I recommend:
God of War series, The Last of Us, Uncharted series, Fallout series, Dishonored, Assassin's Creed series.

tomb raider ftw

A game you can't get on Steam, but I still highly recommend you pick up on GoG is Clive Barker's Undying. Extremely underrated and essentially plays very similar to Bioshock. However, the game predates Bioshock by a good 5 or 6 years.

Aw yesss. Here comes a list.

Slightly listed in order (most recommended games for you are first):

Tomb Raider (2013)
Sleeping Dogs
Far Cry 3
The Elder Scrolls V: Skyrim
Saints Row: The Third
Just Cause 2
Half-Life 2
F.E.A.R.
Spec Ops: The Line
Deus Ex: Human Revolution
Mafia II
Max Payne 3
Batman: Arkham City

SKYRIM

Give a try to Metro Last Light or Metro 2033, I love the designs and story, but if you want you can wait for the Redux versions.
Also try Wolfenstein: The New Order, you won't regret it.

Portal & Portal 2
Spec Ops: The Line
Limbo

Tomb Raider, Batman Arkham City, Deus Ex Human Revolution, the Assassin's Creed games, Watch_Dogs, Metro 2033 games, Half-Life 2, Saints Row The Third and IV, Skyrim, Crysis 2 and 3's campaigns.

Assassin's Creed games and Thief games ;)

Pinball Arcade and Pinball FX2

Loved Bioshock? Don't mind old games? Why not go back to the beginning and play System Shock 1 & 2 (At least 2, since it's easily available...)

Deus Ex series are similar as well. The original is still a great game, just a bit ugly these days. (DE2 has a good story, but being an XBOX port, it's a tad painful to play...)

If you enjoy blowing stuff up, Just Cause 2 is pure almond joy!

Sleeping Dogs.

Fap.

sorry.

Fallout 3/NV

仙剑奇侠传 (Known as the Legend of Sword and Fairy and Chinese Paladin, but I'd translate it as Tales of Celestials, Swords and Legendary Heroes)
新仙剑奇侠传 (New The Legend of Sword and Fairy)
仙剑奇侠传二 (The Legend of Sword and Fairy 2)
仙剑奇侠传三 (The Legend of Sword and Fairy 3)
仙剑三外传 问情篇 (The Legend of Sword and Fairy 3 Waizhuan: Chapter Regarding Love)
仙剑奇侠传四 (The Legend of Sword and Fairy 4)
仙剑奇侠传五 (The Legend of Sword and Fairy 5)
仙剑奇侠传五前传 (The Legend of Sword and Fairy 5 Prequel)
轩辕剑贰 (Xuanyuan Sword 2)
轩辕剑外传：枫之舞 (Xuanyuan Sword Waizhuan: Dance of Maple)
轩辕剑叁：云和山的彼端 (Xuanyuan Sword 3: Beyond the Clouds and Mountains)
轩辕剑叁外传：天之痕 (Xuanyuan Sword 3 Waizhuan: Scar of the Sky)
轩辕剑肆：黑龙舞兮云飞扬 (Xuanyuan Sword 4: Black Dragon Dances and Cloud Flies)
轩辕剑外传：苍之涛 (Xuanyuan Sword Waizhuan: Billow of Grey)
轩辕剑伍：一剑凌云山海情 (Xuanyuan Sword 5: A Sword Above the Clouds, Shanhai World Sentiments)
轩辕剑外传：汉之云 (Xuanyuan Sword Waizhuan: Clouds of Han)
轩辕剑外传：云之遥 (Xuanyuan Sword Waizhuan: Clouds Faraway)
轩辕剑陆：凤凌长空千载云 (Xuanyuan Sword 6: Phoenix Soars in the Sky and Millennium Clouds)
Upcoming: 轩辕剑外传穹之扉 (Xuanyuan Sword Waizhuan: Gate of Heaven)
剑侠情缘2
三国赵云传 (Three Kingdoms: Legend of Zhaoyun)
三国赵云传2：纵横天下
流星·蝴蝶·劍 (Meteor, Butterfly, Sword)
古剑奇谭
古剑奇谭2
幻想三国志 (Fantasy Records of the Three Kingdoms) series
笑傲江湖 (The Smiling, Proud Wanderer)
天河传说
金庸群侠传 (Heroes of Jinyong)
武林群侠传 (Heroes of Wulin)
三国群侠传 (Heroes of Three Kingdoms)
秦殇 (Prince of Qin but it actually means Qin Mourning)
刀剑封魔录 (Blade & Sword)
新神鵰俠侶 (New The Return of the Condor Heroes)
圣女之歌一人鱼的新娘 (Heroine Anthem 1: Mermaid's Bride)
圣女之歌二撒雷母天使 (Heroine Anthem 2: The Angel of Salem)
Upcoming: 圣女之歌三 (Heroine Anthem 3: ?)
天使帝国 (Empire of Angels)
新绝代双骄 (New The Twin Heroes) series
风云 (Fung Wan) series
风色幻想 (Wind Fantasy) series
大富翁 (Richman) series
小李飞刀 (Legend of Dagger Li)
雨血前传：蜃楼 (Rain Blood Chronicles: Mirage)
致命武力 (Fatal Force - it's not that weird game you can find when you search for it on Google D:)

Monster hunter 3 ultimate (Wii U)

A lot of people soured on LA Noire because it looks like a sandbox game (even though it isn't), but I found it to be really engaging. The extra cases were nice too, so I'd recommend getting the Complete Edition on sale sometime.

Bethesda's games (Elder Scrolls, Fallout) can provide an obscene amount of gameplay, but I suppose it depends on how much you like exploring. Sometimes I want something more focused, with a stronger narrative, and other times I want to just wander and explore (like in Betheseda's games).