Peggy Nash or Brian Topp, both are the most left-wing and easiest to attack in my opinion. Neither of them have impressed much in the leadership debates so they'd have trouble going toe to toe with both Harper and Rae. Polling has shown neither of them would do well in Quebec, particularly Nash. Topp could possibly lose a by-election in Quebec, and who knows what might happen with Mulcair and the Quebec wing of the party if he does't win.
Thomas Mulcair's win could also lead to a divided party, which would also be very beneficial for Harper.