Best 5:
Siam (I really, really hate this guy and his fat, stupid elephants)
Greece (Large empire and insane city-states)
Russia (Expands early, always strong in science)
Maya (Often does everything well)
America (Usually pretty strong too)
Worst 5:
Songhai (Never does anything good)
China (Was decent before BNW, but now she always remains tall, and is rarely a threat)
Mongols (attack city-states and nearby civs, and either gets really good or falls. Usually falls)
Polynesia (AI can't use their ability even in archipelago. Also, moai sucks. Why use that precious land for culture in island map, and not enough coast on pangaea)
France (Napoleon goes to war early, but rarely succedes)