It's impossible to say which PC game is the best, because there are millions of them. There sadly isn't just one game that everyone can agree on to be the greatest.
And, whoever says the Call of Duty series is the greatest obviously hasn't played too many other first person shooters.
Is there any specific genre you're looking for?
EDIT: A few great games on the cheaper side (some are a tad underrated):
S.T.A.L.K.E.R
Postal 2 (3 should be out sometime in the future!
)
Team Fortress 2
MineCraft
NecroVisioN
Baldur's Gate I and II (one of the forefathers of the RPG genre, in my opinion)...
god, the list goes on.