Definitely open world sandbox and RPG games, pretty much any Bethesda or Rockstar game.
Love me some Skyrim, Fallout NV, GTA SA, GTA IV and Minecraft.
I also like things like Farming Simulator and Rollercoaster Tycoon.
I suggest just going to amazon, they have quite a lot of sellers that offer a wide range of computers. I found a few decent ones at low price and good specs. Try searching for gaming pc there, I know it sounds vague but it works.