Well it’s key to keep in mind the difference between “Is X the best” and “Is X the best *and* meets some other minimum criteria”.
If you define anti-meta as somehow exploiting weaknesses in meta strategies (as opposed to just overpowering them) then at least you can rank all civs along this criteria.
Italians or Bohemians are probably the best at this.
The name of their games are basically specialized counter units, certainly more than other civs which mostly rely on timing (e.g.
Turks, Portuguese) or brute force/eco (e.g.
Bengalis, Poles).
If the criteria is beating all other (ignoring themselves) meta civs reliably, then that list is probably pretty short and you’d probably have to have Turks of Portuguese as the best.
A mix of both would probably have Bohemians at the top.