Cell stage is the best, hands down, end of discussion. The only way it could be better is if it was multiplayer, had more cell parts and was actually endless. You could just play it for hours, and it would never get boring. Land phase would've been fun too, but I remember spore originally planned for you to eventually become so big in cell phase that, before you developed lungs, you developed gills and would do a version of land phase underwater. The fact that that never happened just sucks.
The reason why its so much fun is because as you play, you watch as these giant behemoth monster cells who simply tower over you constantly, eventually become tiny meat snacks that even become too small to notice they're being swallowed. Multiplayer cell phase would most certainly be the shit in terms of anything ever made. Its simplistic, easy to control, and unless they release Fable 2 as pc game as well, it would be the best thing on the PC until Diablo 3.
Maybe I'm exaggerating, but if anyone else agrees with me on the cell phase thing, please say so. Its just a lot of fun.