Hmm... I'd hesitate to classify Portal as a game in the same group with something like Fallout3. Its a puzzle/problem-solver 'game' yes, but so drastically different gametypes I don't see how it can be compared to something like Fallout3 or Call of Duty.
Portal is an excellent "Brain-stretcher" for sure.
Fallout3 - I'm completely addicted and in no hurry to finish quests - just wandering around is too interesting.
Call of Duty 4 I have managed to avoid so far
Same with Spore, World of Goo, Left4Dead, & Mirror's Edge.
I do find 007 Quantum of Solace to be challenging, though as reviews say the storyline is disjointed when compared to the actual movie.
Legendary, which got huge hype and downright lousy reviews is not quite as bad as they say.... graphics are unimpressive, but it certainly keeps you moving, no standing still. Its a time-killer, but for the money there are better ones out there for sure.
Tomb Raider Underworld I have but have yet to start it
Dead Space - just started, very interesting - not far enough into it to voice an honest opinion.
I enjoyed Crysis - Warhead, but it was too short - such is a sequel however. Same with the F.E.A.R. and Half-Life installments.
Theres more but I'll stop there for now......