It depends on where you want to draw the line of "North America". It also depends on if you mean "Native" species. The largest snake in North America right now is the Burmese Python. Not native, but clearly established in a small area.

If you mean largest native snake north of the Panama Canal... that would be boa constrictors.

If you exclude central america, probably Drymarchon melanurus (Cribos and Indigos).