Honestly it needs to be divided because single player shooters and multiplayer have very different designs and goals, its why one tends to excel at one part and not the other, especially as you get more and more recent.
However for multiplayer specifically
God I wish someone would take anouther crack at this, the high speed mobility mixed with weapons that tend to be devastating but require skill to land hits is just fantastic and never gets old.
For single player its got to be...
Just Half-Life but better in every way, more enemies, more weapons, more interaction. If Half-Life cemented FPS as the most popular genre Opposing Force perfected it.