Far Cry 2
Game has many obvious shortcomings that make gunfight not a good experience; enemies with more realistic sight range (ie: if you can see them by the horizon, so can they), enemies that always know your position once in alert phase, really bad walls that can be shot through (although to be fair those are thin alluminiums), weak and rather inconsistent guns, and also drab graphic + enemies wearing dirty casuals or armor that make them hard to spot, and so on.
Playing that game isn't something I say fun in conventional sense, but from time to time I still boot it up because there's very few fps that give me this indescribable experience like FC2.