Red Faction 2 was bland. It exploited it’s gimmick one too many times, and had no redeeming value.
The Hitman series was enjoyable, as each one got better. Still not perfect, but fun none the less. Thief 3 was a fun game. Pretty graphics too.
Medal of Honor and Call of Duty were both fun games to play. But if you want fantasy/future, feel free to skip them.
Skip Halo. Halo was good for the X-Box because it was the best FPS shooter ever released for a console. But for the PC, it was mediocre. Anyone who plays PC FPS knows that the stuff everyone was so excited for in Halo had already been in plenty of FPS before. As well as every other game did not have such crappy coding. The PC port was terrible. In all honesty, skip it.
The Marine mission in Alien Versus Predator 2 was excellent! Scary, and gives quite the rush. Honestly one of my most enjoyable single player experiences.
Delta Force: Blackhawk Down was fun, and requires patience and skill to play it well.