The 747 express bus is quick, cheap and efficient. It waits for you at the international arrivals area of the airport, there are plenty of ticket vending machines for you to buy a ticket
$9 gets u a ride on the 747 and the rest of the STM network for 24 hours
$18 gets you a three day pass
If you arrive on Fridays after 6pm you can purchase a weekend pass for $12 and it is valid on sat and Sundays.
Most buses have free wifi and luggage racks, sometimes because of demand a regular STM bus without the perks will be used.
Ride takes about 20 mins to Lionel groulx metro and about 35 mins to berri uqam metro.
Note: when going to the airport during rush hours give yourself extra time as there is never ending construction on autoroute 20