The Incans are straight up fun. I like them the most.
However, the best civs in terms of "OPness" is either:
-The Maori with their absurd combo of the Marae, which gives 2 culture and 2 faith and eventually 2 tourism to EVERY passable feature even if its improved (includes floodplains) + Toa, the best unit in the game.
-Hungary for their stupidly strong CS levy rush. Yes, you can get a non ideal start, like any civ, but virtually no one has any way to defend against an early rush by 3 swordsmen and 2 archers with +5 str and +2 move to the second you unlock ironworking. (the AI city states usually have 3 warriors.) If you're good you can combo this with oligarchy for even more cheese. At tier 2 government you can pick up the gov't plaza building to push levied units to +9 strength, which should seal the deal if getting a free neighbor empire wasn't enough.