Well, you can't really consider them STEAM games. Steam is a platform where you can buy MANY games, you can buy almost every game that there is, with the exception of recent EA titles, which are exclusive to Origin. On Steam, you can buy Assassin's Creed, Call of Duty, Far Cry, Total War games and pretty much anything that comes to mind. If you're, however, thinking about games that are exclusive to Steam, then Counter Strike: Global Offensive is an amazing game that will get you hooked if you're into competitive play, but even if you're not, there are casual modes. It takes a bit to get used to since you can't aim down the sights, but once you get used to it, you won't stop playing it. Another amazing game that EVERYONE has to play is Portal and Portal 2. The original Portal is a classic and has to be played, and it was given out by Valve on MANY occasions, but even if you have to get it, it's very cheap on sales (1-2€), considering that Portal 2 is usually 3.74€ on sales. Portal 2 is amazing too and it can even be played with a (girl)friend.
If you haven't played Skyrim, then what are you waiting for? It's usually very cheap on sales, and it was 3.74€ recently on summer sales for the base game and maybe 10€ for the complete collection, which include all the DLC which are worth playing. I have over 900 hours on Skyrim, so there's that.
Another game you may like is Civilization V, it is a strategy game which also goes for very cheap (7.5€ for complete collection) on sales. I have over 400 hours on that game, and it can be played alone or with and against friends or random players. It's a game where you are building your, you guessed it, civilization, and there are plenty of civilizations and historical persona to choose from, ranging from Alexandar the Great to Washington.
Want something free? Be sure to check otu Team Fortress 2, which is a free to play game, which will suck you right in if you like online shooters. I have over 500 hours on that game, but I've stopped playing it a long time ago. Another free game that you may enjoy is Dota 2. If you like League of Legends or similar games, then give Dota 2 a shot. I used to play LoL a lot before, and I occasionally play a few matches, but Dota 2 is a lot better in many ways, from graphics to gameplay mechanics.
Also, if you like WW2 games, be sure to check out Heroes & Generals, which is a free to play online WW2 shooter with a ranking system. You unlock new weapons, vehicles and even different classes to play as as you progress through the ranks. You can be a soldier, a tank operator, a pilot and recon (sniper units, basically). It's a cool little game and should give you a few days of fun, if you're into that. I've got plenty more where that came from, if you're interested.
