Which FPS series has the best single player campaign?

[TWRoO]

Dallinor said:
Half-Life by a country mile. Seriously, it's in a league of it's own.

Other decent FPS campaigns are Timesplitters 2, Halo 2, Killzone 2 and MW2.

Black and Tau Fire Warrior were also quite enjoyable.

I was gonna mention TS2 but having not played the others I didn't know if it is good as a series.