Even after all this time, Portal 2 seems to be the game that's still the one closest to my heart. The story, the characters, the gameplay - everything was executed so incredibly well, so much so that over the years I have to give it another playthrough every once in awhile. Truly a memorable and timeless experience.
Similarly, for a bit of a newer (and indie) game, Iconoclasts is an absolute masterpiece. I hope one day I'll be able to create even a fraction of what konjak has done.