I love how you have so much options when designing vehicles / buildings etc. Expanding the parts creates even more parts and so on. Also the humourus bits you encouter every now and then make this game very enjoyable.
Cell stage: Actually had alot of fun with that, laughing at cells that used to chase you arround but are now on your "things i like to eat" list.
Creature: Wandering arround freely, running from epics, making friends to fight foe's, wonderfull, seeying planets etc in the night sky, beatifull.
Tribe: Gathering resources, outfitting your creatures with tribal style clothing, domesticating creatures, very fun stage.
I had a great laugh ( well afterwards, not at the time...

) when a UFO beamed up my domesticated creature.
Civ: Designing all your things is great, especially with the grand variety of parts. So are the super powers and different ways to choose from when you capture / convert / buy a city. Great stage aswell.
Space: Terraforming, Terraforming, Terraforming.