#1 Huns -- Unbeatable on land maps. Unlimited Horse Archers too strong.
#2 Polynesia -- Unbeatable on water maps. Free movement anywhere = superpowers. Get every ruin on the map, plunder all Cargo Ships, swarm Galleass wherever you want.
#3 Egypt --Chariot Archers are the fastest Domination unit around. War Chariots = better Chariot Archers
#4 Shoshone -- Composite Bowmen before Turn 10 = totally unfair. Start taking capitals by Turn 30, get Range and Logistics before anyone else, and never let up.
#5 England -- There are several other civs that are better on land, and a couple more that are better on water. But I'm giving England bonus points for being great at both.