A no-brainer, to be fair. Half-Life 2 is the daddy of first-person shooters and arguably the best game of its era. Surely you haven't ignored it thus far? If you're still not convinced, download the show-off level Lost Coast or the main demo - they're free. Lost Coast in particular is also stunningly gorgeous if you have modern hardware.
Above: Play it now or they stop smiling and beat you to death