Train Valley [Steam store page]
Starts out simple, but to be really good at this game you have to be able to multi-task incredibly accurately and quickly. That just isn't me, but the game is well designed and will be fun for people who love the challenge of optimizing their strategy and reaction times to climb the leaderboard.

Deus Ex: Human Revolution - Director's Cut [Steam store page]
While the story is weaker than the original, this FPS-RPG still offers many viable playstyles - and a visually stunning world. And the Director's Cut does a slightly better job of integrating the DLC, although they're still sort of an independent experience.

You also get a few other perks like developer commentaries, although these don't have any visual indicators to show where they are, so they're hard to locate and activate as you're playing. The redesigned boss fights are a nice touch though, and definitely a welcome change compared to the originals.

The best choose your own adventure I've played - the choices feel important and engaging, and there are enough features that it doesn't feel like a book

For me this game fell somewhere between Legend of Grimrock and Legend of Grimrock 2 - not quite as good as the open-world of the second game, but Vaporum's voice acting made it more of an engaging experience than the original Grimrock.

Furious Angels [Steam store page]
Simple yet elegant gameplay. This is a great example of how to make a game engaging but super accessible.

The Binding of Isaac [Steam store page]
Decent mechanics but disgusting theme. Enemies can shoot diagonally, you can't. The bad hit-boxes on arrows are frustrating, making yours regularly miss enemies and allowing enemies to occasionally hit you through walls. Made with Flash, low res.

Beat Hazard [Steam store page]
An interesting mix of music and gaming - enemies generated by songs. Disappointing that they charge extra for multiplayer (DLC - BH Ultra).

Left 4 Dead 2 [Steam store page]
Even though Killing Floor doesn't have a story line I still prefer it for co-op fun. This is incredibly easy, and the scripted way the special zombies pop out next to you is annoying unless you have the maps memorized.

Frozen Synapse [Steam store page]
A challenging turn-based squad tactics game, though sometimes the previews are completely wrong. Sadly, game invites pop up even when you're marked unavailable or playing single-player. You must completely exit a level to adjust resolution or sound.

Defense Grid: The Awakening [Steam store page]
A carefully balanced tower defense game with excellent voice acting that weaves an interesting story throughout the campaign.

It looks nice and is enjoyable, but could use some tweaks especially at the more advanced levels. A bunch of enemies in a row can tag team your resources quickly down the line from quite a distance away - which feels a bit cheap at times, especially since the resources travel much faster when rubber-banding to an enemy unit.

It also would be nice to have more control over tower targeting priority, or even just a manual targeting mode. For example, in the later levels you want guns to tear the shields off targets before the lasers attack them - but the guns stay focused on a target until it is completely dead even if they aren't doing effective damage.

It's still one of the better TD games out there though, excellent polish and balance.

Atom Zombie Smasher [Steam store page]
Fairly easy on default, yet nearly impossible to win without using the chooser or combatant modifier - since it often gives you a poor selection of mercenaries. The balance could use a bit of work, since chooser lets you faceroll - but overall fun.

Minecraft in 2D, but with good combat and bosses. You can make a large variety of weapons and potions, and there are sky islands, dungeons, and the underworld to explore. The 1.1 update adds tons of content and improves the game considerably.

Age of Empires II HD [Steam store page]
One of my favorite RTS - though it does suffer from the rock/paper/scissors syndrome once you know the game well enough. Age of Empires II: Conquerors Edition was a great expansion to the original game, and it's bundled into this HD remaster.

The main benefits are that this runs nicely on high resolution - the original was locked at something like 1024x768, and this also has a few tweaks like allowing higher unit maximums - raised from 200 in the original to 500 in this remaster.

I'm normally not a huge fan of re-releasing old games, but this one was such a classic, and the small tweaks they made improve the experience without ruining the nostalgia.