Stalker, shadow of Tsjernobyl
The addictive, I had to finish all the endings. I wanted to see every old building and explore for goodies. Best sp experience in ages.
The annoying, I had to mix 5-7 minimod files together and add my own code to them before I got the sp experience I had been looking for. The retail version is godawefull, yet if you tweak some of the lame stuff out ( bandits spawning constantly, guns not accurate, silly armor/guns wearing down system ) it's a really fun game. Too bad the mp was poorly optimised, because when there was minimal lag it was actually pretty fun.