[Groxx]

Half Life in particular strikes me as a surprising gap in the hall of fame list. And if you look carefully at what little is exposed, there's a lot of information that's implied, and a lot of mysteries that will (hopefully) be revealed in later games. You can practically play the game on instinct alone, and it's intriguing if you look at it carefully, it's brilliant. Heck, the whole series is like this, though HL2:E1 is a bit gimmicky gameplay-wise.

Portal oozes design-awesome, it probably belongs in the list too. The biggest detractors I can see is that the world is pretty small, and thus the story and characters are pretty narrow (though done very well), and the puzzles take away from that world somewhat (they're just sort of "there", they're only part of the story because you have to go through them).