Batman: Arkham Asylum. I was never a fan of superheroes, but after hearing so much about it, I finally decided to buy it when it was in a humble bundle (I actually beat the average on that bundle). Not only was the gameplay great, but it also opened the door to superhero stories in general, and now I'm reading Injustice: Gods Among Us every Wednesday (well, it's currently over until May but I'll be there when it starts again).

I was on the fence about getting Terraria at all for a long time and ended up loving it. Have over 120 hours into it on Steam. Great, open ended game that has a lot of room for actual creativity that most video games don't. And the Metroidvania style action is excellent too.

Also, going back in history, I got RollerCoaster Tycoon for free as part of some promotional giveaway many years ago on CD-ROM. Thought, well it's probably rubbish but I'll give it a shot. My first game session lasted 4 and a half hours. It's a top game, and a really good series. I wish there was a new version now.

The Game of Thrones RPG.

I got it in a bundle and expected it to be horrible, but I enjoyed it. The game gets off to a slow and bumpy start and the combat could have been better, but the story was quite entertaining.

to the moon and half minute hero

Squishy: The Suidical Pig

jk, Tabletop Simulator, Gods vs Humans, Lichdom: Battlemage

Awesomenauts : I played for the Playfire rewards, I have it from a bundle. But this game is awesome it's like a Dota 2 but in 2d. A moba in 2d. It's very fun.

Skyrim

One Finger Death Punch - thought it was a bad game,but then i played it around 4 hours :)

Star Wars - Jedi Knight II: Jedi Outcast - just played it one more time, after a years (maybe 7th time in my life?)
I knew this is great game but I was not prepared to meet so polished, complex and still so addictive game. evergreen.

Jet Set Radio - I expected nothing, I got lots of superb fun in type which currently is hard to find on the PC.

Mark of the Ninja - I expected Shank with some "extra stuff", I got 10/10 game.

DeathSpank - meh, so "funny" clunky fake-rpg. let's try... and bam! so funny casual rpg! :-)

Front Mission Evolved - game is mediocre but I spent great time with it

Spec Ops: The Line.... When I started playing it, I didn't know anything about the game. At first I thought it was just going to be another dull shooter... look those are bad guys, hide here and shoot them in the head to win. But as I progressed through the story ... well I don't want to spoil it for anyone but the story is the real attraction to this game. The game play itself isn't awful, but its nothing that hasn't been seen before.

I foolishly didnt think I would like Metal Gear Rising since it was an action Metal Gear game, but it ended up being one of my favorites in the franchise, and one of my favorite games period

Mount & Blade: knew next to nothing about it, picked it up on a steam sale recently, and haven't been as impressed by a game in a long while.

VVVVVV. From the video/screenshots I wasn't expecting it to be that good.

Hero of the kingdom
Hector
Murdered souls suspect
The cat lady

Mafia II
The Forest (Was surprised how much you can do so far)
FarCry 3 (After 2 I didn't think it would come back)

Bastion
i started it just to get the trading cards and ended up finishing it :D
awesome music and cute story

Lately, Dying Light. Hugeee Surprise.

Spec Ops: The Line
I'm really not a fan of shooters, but this one blew my mind.

Remember Me (has 65% on MetaCritic, found it better than the Batman Arkham games); Brink of Conciousness: Dorian Grey Syndrome (an incredibly good thriller as a hidden object puzzle game); and way-way back, Serious Sam (never liked FPS games, still don't, but Serious Sam and UT got me hooked somehow).

Knytt Underground
Rogue Legacy
Saints Row 3
Tales of Monkey Island-as a fan of the originals I wasnt sure this game would live up to the series but it definitely did.

Teleglitch. A wonderful game which stays too often unnoticed.

I recommend Diehard Dungeon. It's absolutely unknown game, combination of Zelda and Binding of Isaac.

DOOM. I expected it to have aged horribly, and as I have very little in the way of nostalgic ties to it, I mainly just wanted to have at least played the full version. Turned out my expectations were wrong, and DOOM remains an excellent game.

Yet Another Zombie Defense

its also very cheap, try it, wont regret :) http://store.steampowered.com/app/270550/

Crusader Kings II. I was thinking it would be one of those bad 4x games. Steep learning curve too. Thankfully Paradox gave me a week to learn it...then I bought it.