Freeciv is great, but Unciv is the more interesting one to me. Excellent OSS mobile implementation of Civ V. I've burned many evenings away on my phone. The GUI is a bit strange in some spots, but otherwise it's a very nice civ game.
I too prefer Unciv. The mod selection is excellent and includes all of the expansion/DLC content for Civ V, but being able to play it on Android is the real clincher for me.
