[Alneyan]

In KOTH (or similar games), I go for Rock/Oxygen to try having the same atmosphere as the other player. The point is to avoid giving away my breathers, which is something I love to do.

Otherwise, if I expect to do some population trading, I go with Rock and Hydrogen or Carbon Dioxyde. I pick these uncommon atmospheres just to have an edge when trading population, since there will be fewer such Empires, and I could even be the only Empire with one of these atmospheres. It rarely works, and does not really matter, but since you can trade population around, it amounts to the same in the end.