Choosing the best MP game from these would be easy for me - it's BF2 for me.
The game with best SP/MP ratio as I said would be COD.
But, the best SP is challenging to choose - HL1 comes popping to my mind, I really liked the story, and the feel of the game... but I really wouldn't know what to choose as the best SP game.
HL 1 & 2, Far Cry, Halo 1 & 2 - I just cant make my mind on one (Doom 3 had great atmosphere to it but the fear goes away after a few levels, and the game loses much with that)
HL2 had the driving sequences that tended to go a bit boring, and I didnt like Halo 2 as much as I did the original Halo (although they would win for best musical score from these games

).
FarCry just impressed me with it's graphics and the whole level size/design, although it wasnt that much special in any other part.
But I think I would go for HL1, it's the first FPS that I really enjoyed playing, great story, good atmosphere, and I still remember most of it.