Txurce
Deity
What I've found hardest to deal with on Immortal is AI ICS. I have been at war with Greece almost the entire game, and half his strategy seems to be to walk up a hoplite with a settler near my territory and plant a city. They are so close together that I suspect it's a war strategy, rather than expansion as usual. If you don't have iron - I only just acquired it - good luck taking a size 1 city without a concerted effort... which you may not be able to mount if you're fighting the war at the "old" front, against an AI whose tactics have improved to "halfway decent."