I chose Rome. Perhaps I'm just a little biased, but they've always been my favorite civ, on all occasions. Anyways, I do tend to lean towards imperialism in RoN, so the cheaper forts/towers really helps me. And the free units.
I have only played as the Incas, Nubians, British, Romans and Egyptians so far. Of those my favourite is the Egyptians - the farms generating wealth as well as food is handy but the cheaper wonder costs and the ability to build two per city is a real killer.Enabling me to beat the comp. on tougher standard to a wonder victory. I'm still getting my ass handed to me with the other civs at that level - so its the Egyptians for me.
My favorite is France. The healing supply wagons are really useful and so is the free lumber mill. However, they are lacking a bit in UUs. My 2nd favorite is the Russians, followed by the Germans in 3rd.