[Lupusman]

I once tried to figure out which would be best. So I started a few games with ancient race to see the entire map, and counted the frequency of every type of planet, and atmosphere in a mid-life system. Then I figured in the number of facilities that each would accomodate. I don't have the numbers in front of me right now, but what I got was that rock/none or gas/hydrogen are the best combo's. There are a few other close ones, but those are the highest.
Of course this doesn't figure in gameplay factors such as the ability to concentrate on large gas planets. With rock/none, you can spread out much more, but need to spend more on colony ships, and loose defensive capabilities due to the spreading out.
You can also figure in that later on with gas, research and intel centers can be helped with their percentage increasing counterparts (moreso than rock or ice), but usually by that time I already have the other types of colonies.