Actually, it depends on the airline:
Air Canada and Skyservice (as well as the YYZ emergency action plan, see below) list North Bay as their official diversion airport (though they often dont end up there, Skyservice did divert 2 A320s to YYB last year, apparently not a good experience for the pax)
Air Canada Jazz list London then North Bay
Westjet list Hamilton and Sudbury
United, Delta and USAirways list Rochester NY
Continental list Cleveland
Northwest and MidEx list Detroit
Air Transat list Ottawa
Most foreign airlines list Montreal Mirabel or Ottawa
Note: these are the official planning alternates, they do NOT have to be the ones used in the actual flight plans of each flight. However aircraft must carry sufficient fuel to reach North Bay and for 2 approaches in addition to 3 at YYZ and all the other required extras, regardless of airline as it is the airport set in YYZ emergency action plan.
The reason YYB is used is its a 10000'+ runway with Cat 2 ILS and low traffic.
However North Bay lack customs so airliners coming from international destinations will avoid it if possible (see skyservice A320s above).
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.