I LOVE the Cell and Space stages. The cell stage is fun and I like that way you can actually see the world around you change as you grow and get closer to the shore. I always feel guilty killing those first cells you meet as a predator too - the ones with the line of eyes. I think it's the way they squeel as they run away
The Space stage just keeps me engrossed, and there's plenty of options. Expanding your empire, war, terraforming, exploring, buying tools, trading, doing missions, meeting alien races, genetic engineering, the list goes on.
I love the creators and the endless possibilities they bring. The animation with the creature creators is spot on too.
I like the way every time you turn on the game you see something new thanks to the content downloads.
And finally, I like the concept of evolving a species from a single cell to a massive spacefaring empire, and the way you intelligently design them all the way.