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What is the toughest bot ever? (excluding stockfish and the others?)

I'd say Mittens takes it. Mittens' Elo is estimated to be 3600 — nearly equivalent to Stockfish. Furthermore, GothamChess once simulated a game between them that was actually pretty close until the end (Stockfish got the dub).
There have also been simulations of a game between Stockfish and Martina, none of which were even close (again, Stockfish won them all), so that leaves Mittens as the champ, probably.
For me, it is either mittens or martin's wife. What about you guys?