If you could only pick one.
ooh...
So tempting to say Saints Row 1/2 cos I keep going back to them, or Half Life or Goldeneye (the definitive PC and console shooters) but overall I think I'll go with the one that made me a proper rather than casual gamer even though I haven't played it in a dozen years cos it won't run on newer machines (and so desperately needs a remake, especially a console version):
X Wing
(maybe with TIE Fighter by extension)