hmmmm probably HL1 first, jus because that was one of the first games that i ever got obsessed about, i long for the days of roaming black mesa again, plus u gotta pay respects to the godfather of modern good games haha
then HL2, basically because when i started it, i cried tears of joy when i saw the G-Man, then when i finished it, i pretty much shit myself and was liek OMG OMG OMG OMG this was AWESOME!!!!!!!!.
then Portal becasue it made me do the same as the above game, and i say it everytime i play it.
the episodes were amazing, great plots and all that jazz, but a lil too short, plus they cant beat the original HL2