Steam is home to more games than any sane person could conceivably play. Playing them all would send you insane. Which is perhaps why I'm focusing on the quirky and oddball titles that you should be playing.
You know the format by now, right? Yesterday, I ran down the Wii U download games you should be playing, and on Monday Mark ran through his personal top 5 XBLA games. These are very much personal choices, and you're welcome to disagree, although clearly you'll bruise my delicate little ego if you do so.
Take the idea of Minecraft, drop it into a world that looks like it was sketched on the back of Tim Burton's skull, and you've got Don't Starve. Never has a game title been more cruel, as you almost certainly will, and quite a few times. Still, there's fun in perseverance, and even more fun in discovery as you work out how all the parts of Don't Starve fit together.
I've finished Hotline Miami numerous times. I've worried about the effect that it's having on my brain every time I do so, because I cannot think of a game that just… feels… as violent as this one, even though I've played games with equal and often more technically violent content within it. That's a special achievement all of its own, but it's also married to a game that's intensely playable; while enemy AI within the game isn't up to much, it's your own intelligence you'll be questioning after rushing into a room full of heavily armed men armed only with a pool cue… and your brilliance shortly thereafter when they're lying on the floor next to what used to be their brains.
Super Puzzle Platformer Deluxe
The thing with Steam is that there's JUST SO MUCH CONTENT, so this pick really represents what I'm playing -- almost obsessively -- right now. Super Puzzle Platformer Deluxe is Tetris… but you've got a gun. And the pieces want to squash you. Also, there are chainsaws, and everything is hectic, and an incessant tune plays. Also, sometimes, you are a Ninja. Yeah, I think that fits the definition of "quirky" pretty well.
Tiny and Big: Grandpa's Leftovers
Let's see… you're chasing after a pair of magic underpants. Yup, that definitely fits the quirky side of things. You're also armed with a ray gun that allows you to slice and dice the world as you see fit, so it's kind of like Metal Gear Revengeance, only if Raiden was suddenly underpants obsessed… which, actually, he might be. But I digress. Tiny and Big: Grandpa's Leftovers has a unique aesthetic, some excellent musical backing tracks and a nicely open structure that makes you want to experiment… even when you get crushed as a result.
Super Meat Boy
Yeah, I know, Mark picked it for his top XBLA games as well. But I'm not Mark, and I'm not letting him have all the finger-crunching-argh-I-so-very-nearly-made-that-bloody-jump-if-I-just-give-it-one-more-go-oh-go-on-why-don't-my-fingers-do-what-I-want-them-to-what-do-you-mean-its-3am-and-I'm-screaming-at-the-PC-screen fun. And make no mistake, it is fun.