I just want to make sure you know that I don't think this is a definitive list of games. This is a list of games that I like, and think should be played. This list is definitely incomplete and may be updated without notice!
Mass Effect
The iconic third person shooter game with classic role-playing elements and hero themed story line. As the unforgettable Commander Shepard, experience a galactic adventure with your crew, fighting an enemy which threatens all living beings.
Trine
Control one of three characters at any time, using their skill set to overcome challenges ahead. Although this game can be played as co-op with friends online, it's just as fun, and perhaps more rewarding as a single player game.
Dying Light
Quite possibly one of the best single player games that I've played. Graphically superior to many games, with controls that "just work", and a decent story line. Running from zombies is challenging during the day, but when the light dies you may not survive!
Stardew Valley
Do you like farming? Do you like pixel art? This might just be the game for you! Influenced by several popular games from yesteryear, it manages to improve on those classic games and bring a few new things to the table.
Soldier of Fortune
This is a game that's perhaps most well known for being very graphic. The level of violence is high, but the game itself is solid and feels great. If you've never played this and you call yourself a hardcore FPS player, then you're kidding yourself!
Super Mario Kart
Games that spawn many clones deserve to be known as classic. Super Mario Kart not only proved that trend setting racers were possible in the 90's, it also proved that pushing the limits of hardware was a crucial step in mastering any platform.
Red Faction
The game that inspired many developers to take a closer look at geometry to make things more realistic. Trying to imagine games without destructible walls and buildings? More or less every game before Red Faction was like that.
Hitman: Codename 47
Chances are, you've heard of the Hitman series because there have been several games and even a movie spawned from it. The original gave Metal Gear a run for its money in terms of strategy, though it was more cold and callous in execution.
Zelda: A Link To The Past
This title showed that attention to detail and smooth game play were the workings of a truly great game. The world to world shift was brilliant and inspired many developers to consider alternative situations for their games.
Metal Gear Solid
There's really no way you can exclude this game from any praised list. Metal Gear games are always so cinematic and it's hard not to be drawn in when the game play and story are so well conducted. Huh? What was "that" noise?
Final Fantasy 7
Just one of a series of FF games. The graphics in this game, compared to more recent games could be considered low-tech. Graphics are not what this game is about... it's an adventure to experience, with characters that remain with you for years to come.
Sonic & Knuckles
Not only one of the last 2D sonic games that was addictive, it was also another game that showed great innovation. The lock-on technology of this game allowed the previous 2 sonic games to be plugged into the cartridge, providing additional content.
Half-Life
What an epic game! When Valve released this in the late 90's, nothing came close to beating it. The dark and twisted storyline, the no holds barred attitude of the NPCs, the horrific alien creatures and let's not forget the iconic crowbar melee weapon.
Minecraft
Although widely accepted as a multiplayer game, I see this as the ultimate, offline, retro, building and survival game. There's a lot of things to do in this game and the peaceful music compliments it well, despite some of the sound effects being less peaceful.
Age of Empires 2
Is it a building sim? Is it a war game? No... it's both! Such a nice game that required some thought and a lot of patience, in order to succeed. In the year of release, there was nothing quite as polished or fun to play that had the same depth.
System Shock 2
This seems to be one of those games that a lot of people haven't heard of, and that's a shame because it's a great game. It's a mixture of several genres, wearing a first person shooter mask. Brilliant as a single player and even better with co-op!
Terraria
It's a 2D style builder and survival game, similar to Minecraft. The retro graphics give it an appealing look and like Minecraft, the music is part of what makes it great. If you've ever wondered what a platform game mixed with Minecraft would be like, this is it.
Kingpin: Life of Crime
This was such a violent and profane game for the year it was released. It faced much controversy because of the many gruesome deaths, swearing matches, dark and gritty locations, needless explosions, and insane dialogues.
Diablo 2
The legendary hack and slash game that is regarded highly by fans of the genre. Although this has a single player mode, it was also made to be an online, multi player game. The nice visuals and interesting game play make this a classic!
Alan Wake
If you're into games that are heavily story driven and don't require too much investigation to progress, this might be what you're looking for. It's basically an interactive novel, which is read to you as various events take place. The theme is darkness.
Starbound
This is another game that is similar to both Minecraft and Terraria, but instead of just chiselling away at a single world, you're able to travel through space in search of new worlds. It's a very relaxing game, that will steal hours away from you.
The Elder Scrolls V: Skyrim
Quite possibly one of the best role-playing / adventure games ever created. It's popularity has been steadily increasing since release. Thanks to the modding capability (on PC) it can never really be forgotten, despite already having story-line freedom.
Mirror's Edge
As much as many people love to hate EA DICE, nobody can deny that this is a fantastic game, even by today's standards. Despite there being a reboot of this game that has advanced graphics and gameplay, the original still holds strong.
FTL: Faster Than Light
Unlike many games, this one is created with failure in mind. There are no real hand holding moments and if you make a mistake, that's it! Resource management of a starship has never been so much fun, or rewarding when you finally win!
Fallout 3
Perhaps to the disgust of pre-fallout 3 fans, and to the frowny faces of Fallout: New Vegas lovers, it's one of my favourite single player games. Fallout 3 is an open world adventure, focusing on surviving in a post nuclear wasteland.