Heyo

Counter-Strike -> This is a game that will never get too old for me. I have spend hours upon hours in it (This playtime is extremely low considering I have played it on other accounts and non-steam too (Yea, I know, I am a bad person for cracking it, but give me a break, I was like 10 years old). There is not much to say about it, if you don't know what it is about, you are gamer mate.
Terraria -> Funny thing, I actually thought this game was not that good, but considering 2 of my friends were playing it at the time and it sounded like they had a lot of fun, I ended up buying it and not regretting a single penny spent. The game is simply amazing, it has great singleplayer potential, it has great multiplayer potential, even modding is completely supported by developers themselves so that is great (You can play mods multiplayer). New mods come before you complete old ones so you will never run out of stuff to do. It is simply amazing. Game is basically a survivor platform minecraft-like game (blocks and shinanigens). There are bosses, materials, building, designing, dying, killing teammates, trolling teammates...
Keep Talking And Nobody Explodes -> A game that is most fun when played with someone that is not experienced in gaming/that gaming. The point of game is that one person/team can see the bomb that they need to defuse (with a timer), but they do not see the actual manual on how to defuse it, while other person/team can see the manual, but not the actual bomb. This game is pure definition of "The more, the merrier", there simply can't be "Too many people playing".
Age of Empires -> A strategic game that can be playing singleplayer and multiplayer. The more you play it, the better you will become. It is very interesting if you talk to your opponents while playing, especially if the game is diplomacy game (You can change anyone to ally or enemy at any point) and you are talking with allies/enemies while playing.
Magicka -> Don't really know how to explain the game, but a very attractive feature is that friendly-fire can not be disabled which means you can kill your teammates while you are killing the enemies by accident (Or not ?!).
ShellShock Live -> Just look at 2 minutes of gameplay, if you have never seen anything similar you are not a true gamer.
Vessel -> A very complex puzzle game that requires a lot of attention and thinking. Considering I love such games, I loved it and I recommend trying it if you think you can handle it.
Hue -> Basics are same as vessel, a complex puzzle game that requires a lot of attention and thinking.
That would be all. I am way to lazy to write comments for every game on the list so I simply won't do it :P . If someone is interested in my opinion about a game on lists above, feel free to ask and I will be more than glad to give them to you.