No point getting games that you can get on a console IMO, unless you don't own a console.
If you own a console:
Arma II - free to play the standard multiplayer with the Arma II Free on Steam.
Terarria - possibly the best Indy title out there, I prefer it to Minecraft, personally.
Diablo III - despite all I bash it, its has great gameplay, just everything else about it is horrible.
The Elder Scrolls V: Skyrim - while it is on console, playing Skyrim on console is about 10% of the real game thanks to the modding community (you may also want to look into Oblivion & Morrowind)
Thank you and everyone else
S.T.A.L.K.E.R (Call of Pripyat, Clear Skies & Shadow over Chernobyl) - the best single player experience on PC in my opinion.
BTW If you enjoy Arma II Free, I'd recommend buying the Combined Operations pack for it, which will give you access to the whole game and allow you to play the amazing DayZ mod (or just buy Operation Arrowhead and use it alongside Arma II Free if you only want DayZ, but the textures are extremely low quality if you don't own the vanilla Arma II).
If you don't own a console:
Mass Effect 1 & 2 (play 3 if you want more story, as it is a terrible game) - Probably the best Sci Fi game available, although I'm sure plenty of people will argue Deus Ex on that one.
Call of Duty: Modern Warfare - don't bother with any of the other CoD titles, Modern Warfare is by far the best, and easily has the best community of all 1,000 titles.
Left 4 Dead 1 & 2 - although probably better played on PC, i would say that you should only play it on console as it isn't good enough IMO to warrant taking place of a better PC exclusive if you have a console. That is if your strictly looking for
five titles to try, don't let anything thats on a console take that place, unless as I said before, you don't own a console.
Minecraft - again, this is on console too, so only try it on PC if you don't own an Xbox.
Dead Rising 2/2.5(Off the Record) - both games are the same price now, but Frank West is too awesome to get 2 over 2.5!
Hope that helps.