The Swapper! Creepy atmosphere, great gameplay, intriguing story. Also not too long

Portal 2 because it has an amazing story and gameplay

Mass Effect series. Should speak for itself.

You should give EV Nova a shot. It's a space sim where you start out with your own little space shuttle and you progress from there in any direction you want. You can become a trader or a pirate. Join any of the 7 or 8ish factions as they fight it out. Great to play through their storylines. Eventually work your way to owning your own fleet and taking over planets to pay you tribute. You can outfit your ship in many different ways to cater to your gameplay. The community is pretty awesome too having a variety of different mods to choose from too. Overall great gaming experience.

Also, did I mention it has...

it's own free trial?

But there are too many of them in every genre! How can we compare Metal Gear Solid with Fallout or Mechwarrior with Gemini Rue? Or X3 with Starcraft? Or X-com with Hydorah?

Warhammer 40k Final Liberation - Because Commissar Holt.

Star wars knights of the old republic.

The first game was Bioware at it's best. You have a quest that is epic in scope, mini-games I actually enjoy (well besides the racing) and well written dialogue. It also helps that you get a shiny shiny lightsaber. (which is clearly superior then a chainsword)

Mass Effect series, Starcraft series, or both Star Wars: Knights of the Republic games