I'm going to assume that you want a downtown hotel. We usually stay at the Fairmont Le Reine Elizabeth, which is very conveniently located close to the main train station and the subway. It is also connected to the underground shopping pathway. It is not cheap, but you can get specials for about $149/night plus tax, which does not include breakfast. You will definitely need reservations, Montreal is a "destination" city; downtown hotels are often booked up.