Primacide
Settler
I think play style is so critical in determining OP. While I appreciate that the spirit of the post was in reference to generally-accepted strong civs, I have found other people absolutely dominate with civs that I am just meh with. A friend of mine crushes me every time with Babylon but when I play them I do... Just okay. However, when I take Egypt (not an especially dominant civ at higher levels where you get NO wonders, cept the stupid himeji castle) I can't lose. My religion piles bonuses on the already free temple UB and... See y'all in the modern era.
(I will not mention their UU. Nobody should EVER mention their UU.)
(I will not mention their UU. Nobody should EVER mention their UU.)