Guessed by @UnderwaterFunktown
Mind blowing barely begins to describe the feeling of playing Portal for the first time. That mostly applies to the first game, to be fair, but while that one was almost like a prototype, a little experiment, this here was a massive game with a fully fledged out story, tons of mechanics and types of levels, environments, and it's not like it had a shortage of ways to blow your mind all over again. It never ran out of them, actually, because even after the main story, you can just go into the user-made levels and have an infinity of insane puzzles. And there's even a co-op campaign that is also surprisingly great. Honestly there isn't much to say about Portal 2 because it's just so flawlessly good, and everyone knows it. So I'll just leave it here.
