Zouave, when cities renounce the owner, the armies stationed there disappear and the population stays the same, AFAIK. I don't know if improvements stay or not. One or two defensive units appear. I'm not crazy about the rule, but it does make it a lot harder to steamroller a rival civilization.
As to the best civ, I like the Babylonians so far, mainly because the scientific/religious combo suits my playing style. I agree with Ceredhion that the earlier special units appear, the better, because they allow the Golden Age early on and combine it with a good attack unit, which almost guarantees success in the early war I like to provoke. I also like civs that start with bronze working (like the Babylonians), because then I can research iron working straight away, which helps with planning.
The Persians have a great special unit, maybe the best in the game IMO.
Some civs, such as the Japanese, have a special unit that comes from a "skippable" tech, which is a drawback. Samurai are good units, but I often skip feudalism unless I happen to be at war when it becomes available. Same deal with the Chinese. I can't really see skipping military tradition though because cavalry are too valuable.
The Americans are maybe the worst civ, IMO. I'm not big on the expansionist attribute and their special unit comes very late in the game.