S.T.A.L.K.E.R.: Shadow of Chernobyl, modded with Stalker Complete 2009 (2012 update), a simple ENB and Stalker Complete weapon pack (available through the ModDB page) is really fun. When I first played the game, I was intrigued but thought the game was far too difficult and annoying to get invested in. I learned that the majority of the difficulty lay in the fact that the difficulty settings do not affect the game in the traditional sense, but that it makes bullets do more or less damage to EVERYTHING and on the lowest difficulty, bullets have a 40% chance of simply disappearing.
After learning this (and now that I have a good gaming rig), I went back to the game and cranked that difficulty up to Master. I haven't had so much fun in a long while in a single player game. I mean, I love my modded Fallout: New Vegas and Skyrim, but this is a wholly different experience.