Fallout 3

Fallout 3 is currently my favorite game to play (except, possibly, for TF2).  It is an almost perfect fusion of FPS and RPG.  It even includes two different methods of firing your weapon: free aiming a la usual FPS's, and VATS (vault assisted targeting system), in which you select your target and let number crunching do the damage for ya.

The game is set in the semi nearish future of the late 23rd century.  A nuclear war in 2077 between the U.S.and China has left the world a desolate wasteland.  A small portion of humanity, however, has survived, thanks to giant underground bunkers called vaults, designed to allow people to live through just such a holocaust.  You play as one of these vault-dwellers over 200 years after the great war.

The game has a great atmosphere.  It's timeline follows that of our own world up until about 1950.  At that point it diverged, causing interesting differences.  For example, while we have advanced highly in the areas of computers, they have highly advanced in nuclear technology.  Thus, in the fallout universe, computers are large, bulky, and can display only one color, while the family car is powered by a small nuclear reactor.  This and other similar qualities give the game a unique character.

The gameplay itself is a blast.  As it is an RPG, you gain experience and gain levels, allowing you to improve your various skills.  You also acquire a perk every level, which give you a special ability, such as increased damage with fire-based weapons, the ability to carry more equipment, or higher resistance to radiation.  The combat itself presents many entertaining ways to eliminate your opposition, from the simple combat shotgun, to the high-tech plasma rifle, to the mini-nuke lauching Fat Man.

Overall, the game is quite an experience.