Someone mentioned Planescape Torment. I second that one. Easily one of the best RPGs ever created. Morrowind was pretty good in design and concept, and rocked the first half. It fell short towards the end game. Too bad, it had lots of potential.
If you're an RTS fan, or strategy fan, or both, then by all means do not hesitate one second longer and buy yourself Rome: Total War. It has quickly become one of my all time favorite games. Very strategic (think xcom), very tactical, and even has RPG elements to it. And you could play it 4 or 5 times over again and it be a different game each time. Game of the year, easy.
Shogun: Total War (older, but still good) is the next best in the series in my opinion. That might only be because it was the first one and I played it to death when it came out. So I might have fond memories of it. But I still like it better than Medieval: Total War. Dunno, something didn't rub me the right way in that game. Shogun rocked though.
So yea, Planescape Torment, Rome: Total War, and Half-Life 2.
As for multiplayer action, there has yet to be a game that bests the pure adrenaline, speed, skill requirements, and fun of Quakeworld Team Deathmatch or Quakeworld Clan Arena. Sure, it was the first real mainstream multiplayer first person shooter, but they got something right with that game. The only game that has come close has been Tribes 1. I hate this current trend of realism and slow gameplay personally. I liked Rainbow 6 and counterstrike when they first came out, but man give me speed and skill anyday. I want aim to actually be a factor in game, not BS random scattering. I want to be able to control my charecter in crazy movement and use the enviroment as part of the game. Not just roll down cookie cutter hallways and paths and shoot. Back then, games took skill. Now they just take time.
Tribes 1, Quakeworld, and Action Quake2 will always be the pinnacle of gaming to me until someone comes up with a faster, more skill based game. It's time to start catering to the hardcore shooters again
. Quake3 was too balanced and too mainstream. I have hopes for Q4, but who knows.
Fun > Realism