The default is Emperor for me. I played a lot of Civ5 a while ago and found Emperor too easy, so migrated to Immortal, but after a bit of a break from Civ5 (thanks to Minecraft) I found my play was no longer up to Immortal standard any more, so I dropped back down to Emperor.
Now for G&Ks I've stuck at Emperor, as it's well within my comfort zone and so I can try the new content out, without trying too hard at the game its self.