Here are 3 of my 5 Steam GotY contenders, thought I'd share my initial write-up for now as I ponder what my 5th game would be (hopefully it'll be Styx, Shadowgate, Mordor or Wolfenstein)
Freedom Planet
Freedom Planet is a 2D Platformer with visuals inspired games on the SEGA Genesis/Mega Drive. I had gotten interested in this game after reading up about it in the Sonic Community Threads, and as someone who liked Rocket Knight Adventures and the original Sonic Trilogy, I just had to get this game.
You play as one of 3 characters, each with their own unique playstyle and moves, progressing through 12 beautifully drawn 16-bit stages, each stage with its own theme reflecting Sonic 3 and Knuckles (not the stages themselves, but rather the variety and how each stage stands out from its predecessor and successor.
Suffice it to say that this game has a great soundtrack, and like many games of the era, the boss battle theme(s) are quite memorable, one even having an upbeat tempo that just gets you hyper to battle those powerful bosses.
If I had any negatives to say about the game, then itd be that the stages feel really damn long first time you play them. Each stage would take 10 minutes to complete first time around, but at least the game manages to keep a good fast pace and offers various paths that allow you to complete stages. I wouldve preferred if each stage was divided into 2 acts, as my initial playthrough had me plough through the game one stage a day.
At the end of each stage is a boss, and in some stages there are mid bosses. Bosses in this game can be quite a challenge as you must learn their patterns and time your movement to avoid taking damage, lots of mess ups and you may end up having to restart a stage! It is also one of the few games Ive played this year where I really enjoyed the boss fights, with the other being Ys: Oath in Felghana.
I personally enjoy playing as Lilac more as she reminds me of Sonic and Rocket Knight and I like her moveset. I did try Carol and Milla, but they didnt sit right with me, but kudos to the devs for offering variety to the way you can play this game!
I also like to give this game the honor of having a great villain with a neat novelty to his final battle; where normally the final boss would go into their next/final form, Brevon, starts battling you in his power armor for 2 rounds, before battling you without it; and he hits HARD SON! 1 hit from that sword of his and bye bye 80% of your health!
It would be safe to say that this game is to the Genesis/MD, what Shovel Knight is to the NES, and I recommend getting this if youre a fan of 2D platformers or the Genesis/MD in general.
As a final note, the game has a good story IMO, but you can play in classic mode to play without the cutscenes.
 
Killer is Dead
There were many games I played on PS3 that I wished would get a PC release just to fix their technical and performance issues. This is one them, yet I managed to overlook its performance setbacks and voted it as my GotY 2013.
Killer is Dead is an action game from Grasshopper Manufacture, where you play as Mondo Zappa, who one day, wakes up to find a mechanical arm strapped on him, and is recruited to an execution agency that specializes in eliminating monsters. The game will have you go through various levels each with their own unique design and setting, from a normal house that suddenly transforms into a crazy dollhouse, to fighting enemies and giant eyeballs in a mansion on the moon, to stopping an enraged sentient train turned eldritch abomination in Russia.
You know, typical monster slaying stuff.
The games combat is really simple; Your main weapon is the beloved sword Gekkou, and as you can keep attacking enemies without being interrupted or taking damage, a gauge fills up which increases the damage from your sword and its speed. Mondo can also break an enemys guard or stun them with a punch from his mechanical arm that can also increase the gauge.
In addition Mondos mechanical arm functions as a gun which he can use to dispatch enemies with. By playing Gigolo missions you will unlock additional forms for your mechanical arm to transform into such as a drill or a charge cannon.
Mondo can also block and dodge attacks, a properly timed dodge will allow you to counter an enemy by dashing to them and unleashing a fast barrage of sword slashes while game time freezes around you and the enemy youre countering.
As you progress through the game your health & Blood (equivalent of MP) can increase from items dropped by defeated enemies or found in breakable objects. You can also earn points that can be used to unlock more abilities. Furthermore, completing missions earns you cash which you can use to buy items, mainly for gigolo stages, but there are other items that can be used in-game.
The games structure involves completing missions, with the first 2 serving as tutorial modes, and mission 3 being the intro stage of sorts. You can access missions from a world map hub allowing you to replay the stages/missions you completed on harder or easier difficulties.
In addition to the main missions, you can unlock special/bonus missions that have specific objectives in order to earn cash and points to learn upgrades.
My main attraction to the game is that its by Suda51 whose past games Ive really enjoyed due to their simple concepts and gameplay, and especially the crazy stories and events that occur in the games (did I mention that one of the stages/missions is a mansion on the moon?). I was ecstatic to learn that the game was releasing on Steam, and applying the simple 60 FPS fix among a few others has really made experiencing this game much better.
Gunhound EX
Originally released on PSP, this game was released earlier this year courtesy of Playism Games.
In this 2D action game, you pilot a mech with a machine gun as you progress through stages defeating enemies with a confrontation with bosses and mid-stage bosses. Although a short game, it manages to offer as much variety as it can with its stages, from battling in a landslide from a mountain or infiltrating a high tech base with various traps.
Your primary means of combat is a machine gun that reloads when its ammo runs, thankfully this only takes 1 second, but it helps to time your shots so you get the most damage on an enemy before reloading.
In addition you have a melee weapon you can use to damage enemies if youre close to them. An option in the game will make this melee attack pop up automatically if youre within range. Metal Slug anyone?
Finally theres also another type of ranged weapon equipped on your Gunhound which can deal big damage, but its ammo will take some time to reload after firing.
As stated previously, each stage ends with a boss fight, and Ive found the boss battles well designed for the most part, as they involve learning patterns and dodging/avoiding their attacks.
If theres one gripe I have with the gameplay, itd have to be in any anti-gravity sections, worse is that one or 2 such sections are boss fights! To be fair, the game does offer you a simple method by which to have your gun firing at the boss, but maneuvering around a certain boss was a pain in the ass.
Although you have a lifebar, you cannot recover health, however, you can get rid of your initial coating to reveal the interior armor plating on your mech. Doing so gives you increased movement speed and fully recharges your health, making it useful for critical situations.
Graphics-wise the game is very pixelated despite the in-game smoothing option, and I wouldve liked for it to be cleaner when playing full screen. Non the less the 2D graphics and overall presentation is really good and very colorful.
While a short game, it does have unlockables, earned by gaining points from playing through the game. If you manage to beat the game without dying or using a continue you earn the most points and they go to unlocking extra equipment.
These additional equipment can replace your initial weapons from the Extras menu, and include a flamethrower as well as a shield thatll replace the melee weapon allowing you to block certain attacks.
All in all, if youre a fan of Run n Gun style games like Contra or Metal Slug, you should def. give this game a chance. It also has the benefit of having a good simple story with a good female protag, and despite being a short game, benefits from replay ability cuz whats not fun about piloting a mech and blowing stuff up in the name of justice?