Far Cry 3: Blood Dragon. It's not DLC, it's a standalone game that only costs $15 on Steam. I REPEAT you do NOT need the Far Cry 3 game to play this.
Aside from this, it is perhaps the funnest experience I've had with an FPS in about 4-5 years. Don't watch youtube vids on it, don't look up information on it, just play it. You'll be captivated.
Other than that, I would say the Bioshock series (most definitely Infinite), Tomb Raider, and Max Payne 3 are all really solid games. You can find a lot of other classics too that Steam puts on sale from time to time. Go out and explore different genres. PC gaming is a cut above the rest.