Are you talking about 1 or 2? I always thought 2 was better and introduced a load of mechanics (to me at least) that are commonplace nowadays. This was the first FPS game I played where getting found during a stealth mission didn’t completely ruin everything. And the concept of unkillable enemies that you can only run from.
TLL (Tornado Low Level) on the ZX Spectrum was amazing at the time, and Cyclone was a sort of follow up but I don’t know if it was the same developer or anything.