chieftain - for a normal game. well i used to play it when i was six or seven

but great for 1cc or 2cc peaceful SS strategies. strange AI handling on this difficulty. the great warmongers (mogols, greeks) are failing and peaceful builders (babylon, china, germany) became extremely dangerous opponent in space race and also in nuclear wars.
warlord - still very easy. not much harder than chieftain. maybe ai is stronger.
prince - getting interesting, AI techs up much faster and rapidly build empires in peaceful times. trying to be democratic-economic-superpower often leads to disaster when the greeks or zulus appear with dozens of military units

modern conquests are many more challenging if you doesn't face phalanxes with your armours.
king - really hard. theoretically the AI has no disadvantage or advantage to you, but pragmatically very hard to be on your guard. they are very good at economy. i would say one should give up builder style for warmonger. mongols, russians are pain on this difficulty
emperor - once tried with the germans, finally the indians won. luckily gandhi is very peaceful so he didnt waste me, though he was more than capable of. i dont know how you should win a SS victory here or tech as fast as the AI and have dozens of able military. i was barely hold off the greeks, though i had twice as many cities..
thats all
mine is prince/king, i dont think i will ever be able to win on the mysterious
E 