You choices:

Aerobus: $14 per person one way, $24 round trip. Runs every 20-30 minutes. It stops at a few different hotels in downtown before ending at the Berri bus terminal. On the return trip, you must buy a ticket at the Berri bus terminal or have one from a round trip purchase; they do have a free shuttle from downtown hotels to the Berri bus terminal (call to reserve it).

Taxi: $35 per cab to downtown.

Limosine: $50 per limosine to downtown

Public transportation: local bus to Gare Dorval, commuter train into downtown Montreal Vendome or Bonaventure station, Metro subway if needed to the closest Metro stop to your destination.

Rent a car, drop off at the rental company's lot in the city (keeping it in the city is too much hassle with respect to parking).

http://www.cbp.gov/xp/cgov/travel/vacation/kbyg/