After reading your subject line I stated to think... My first reaction was to think of Half Life. If we are to include all of the Valve games to have come from HL then it has to be up there with the very best.

For single player Call of Duty must get a big mention, the cinematic story line. If you think it's a cheat or a stroke of genius to have you play through the best bits from well known war movies. If you love war movies then you will love Call of Duty, you get to take part...

We also have to mention some other great single player games like Farcry, Project IGI and a game series I have put a lot of time into Medial of Honor.

As for Multiplayer then the simplicity of counter strike, DoD or MoH. How about the total fragfest that is Quake?

I like my fps games to be a little more tactical so I like the big map games like Battlefield series and Novagames titles like Joint Opps the latest in the Delta Force family.