Now that most of us have played it to varying amounts, here's my retake on my favorite stages (and not so):

starting from the bottom:
5.  Tribal is probably my least favorite, probably based on the fact that there's very little or no real customization, as in none of the content comes from any of the editors (apart from the 'outfitter', but that hardly counts)... apart from the creature(s) you made from the previous stages.

4.  hmm...i guess cell phase; while I do like this phase a bit more than a '4th place' would imply, it loses out to the civ stage because of the latter's vehicle and building editors.

3.  Civilization stage (see #4); granted this is a very simplified RTS mini-game, but it's still fun, and hey, it has the vehicle and building editors

2.  space age- another close tie, with the Creature stage, it really only loses out because i haven't put much playtime into this phase yet (i keep starting other civs on other worlds instead), gotta love the space-ship editor  :)

1.  and so this leaves the Creature stage to be my favorite, as (for me) this is really is what the game is really about, evoloving your own species and seeing others' creations much more up-close than you can in the other stages (w/o zooming of course)

just my (2 cents x 5..) dime

Space stage is by far my favourite stage.

Space stage is the most fleshed out and fun stage IMO.

i agree with space age being the most fleshed out, which is why it was pretty-close to it being my favorite, but it 'missed' because, well, frankly the main concept of creating a creature via evolution is no longer part of the game at that point, instead its become completely different (not in a bad way of course).  so to stay true to the intended nature of the game Spore, the creature stage stayed my favorite phase (as in my initial post)

I have to say that I really like the creature stage and the space stage equally, The creature stage is just fun to spend ages on exploring and altering your creature whereas the space stage is so expansive anyway and actually needs a lot of management skills. one thing I don't like about the space stage is that you can no longer reinforce defenses on your home planet, you feel separated from your race a little.