The best place I've found to ride is the Circuit Gilles Villeneuve, Formula One race track on Ille Notre Dame. It is a 2.6 mile circuit with excellent paving and the bikes are divided from the car traffic.
Also, next to the Olympic Rowing Basin is a 2.5km paved road that is smooth, dead flat and protected from wind by a berm. Great place to do intervals as the distances are marked out.