I personally loved Raven Shield. It's very hard, but it adds a certain atmosphere you don't get in other games. Add a brilliant AI, and you got yourself a great game. My favourite moments are for instance sneaking up to the attic on my own, shooting some guy from behind (with a silenced gun, of course) and silently opening the window just enough to get my gun out. This done, I order my team to go across the street to the next house while I cover them as best I can. Both the enemy and my team act very realistic, and I almost forget they are just AI controlled at times. It's awesome, if you like this kind of stuff. Again it's very difficult, but you have a great AI team to help you out.