This is kind of a tough question. There are so many good FPS's.
It takes a lot more then just shooting guns to make an FPS good.
I would say currently top 5 PC FPS's would be:
Battlefield 2
Half-Life 2
Doom 3
Far Cry
Unreal Tournament
Not in any order.
Soon in October (F.E.A.R) is available. Which if you've played the DEMO, should have no problem joining this list.