Legion

At the beginning of this game, I didn't know what was going on at all. Having completed it, I'm still not sure. And somehow, this is the coolest thing about it. You learn bits and pieces as you go along: for the first few moves, all you understand is that you're in danger and you have to hide, then when you explore a little, you come to understand where your enemy is and that you have to attack it, then - when the main NPC gets involved - the nature of the enemy becomes clearer, and so on. I very much got the feeling that my understanding as the player was progressing at the same rate as the player character's (characters'?) - including the fact that I still don't understand exactly what the PC is (are?) (a swarm of microbes? A planetary consciousness? Pratchett-esque Small Gods? Nature itself?) This is made all the better by the fact that the game never tries to spell out the answers to these things.

One of the first things I did when I realised that the PC could move through solid objects was to see what happened if I kept going DOWN - I was pleased to see that the mantle, core, and far side of the planet were all implemented. Details like this make me happy.