From Single Cell to Galactic God, evolve your creature in a universe of your own creations.
To me, this is a sheerly amazing game. While I would say that it's not perfect, it's still great enough to earn a 10/10 in my book. Other than the fact it allows you to create practically anything, it's big. Really big. It's one of the largest games ever made, and I love it in that respect. I wish EA didn't just abandon the game while it was getting good...
This is sort of an ultimate simulation game - sandbox arcade, adventure/exploration, and real-time strategy, all in one. You can customise everything, including your civilisation (not just creatures themselves).
Oh, and was a very active modding community even mentioned? Now it is!