Portal 2 is a perfectly written game. Every character, the story of GLaDOS and the emotion in her voice that gradually changes over the story, Cave Johnson and his lemons, and of course the ending...
Then there's the fact that these happen in a forgotten corner of the world in Half-Life, with the first game happening sometime after the experiment gone wrong at Black Mesa and the portals bringing headcrabs, Vortigaunts, barnacles and the like to the world, and around the time the formidable Combine seize governance of human life. I like to think that maybe Chell wakes up in Portal 1 during the Seven-Hour War...
Portal 1 also is fun because it's easy to bug out and glitch all kinds of stuff, and it's easy to speedrun.