I suspect most accomplished tuners will reply that its not that simple. Generally speaking most will shoot for something between the high 12's and low 13's to 1. But every engine is different and the only way to know is to put it on a dyno and tune for the best result regardless of where the actual A/F number falls.
Give the engine what it wants, not what you think it wants.
Generally speaking at WOT, 12.8:1 up through peak torque, tapering to 13.2:1 near peak power. But Lambda sensors can show different values when compared side by side. Ignition advance will alter sensor readings as well.
Fuels with ethanol require different mixture ratios than without.