For a place with some character, I like Chateau L'Argoat, Sherbrooke West near Berri. http://www.hotel-chateau-argoat.qc.ca/en/ It is at the low end of mid-range prices.
It's a small place, set in two old mansions, well maintained and handy to both the Plateau and the Latin Quarter, with a Metro stop a block away. It also has limited free parking although you will need directions about how to gain access.
However, if you are staying in Montreal itself for several days, you do not need a rental car as public transport is good and parking is scarce.

Not that there's anything wrong with it but Sherbrooke & Berri isn't Sherbrooke West. It's East.

I will second the rec for the Argoat, it's in a great area - close to both the Plateau, and the downtown. If you want to stay downtown, there are a string of large chain hotels on Sherbrooke west - Holiday Inn, Marriot Courtyard and the Delta. They are all 3* with parking garages, and in a good location. However, I prefer staying at a b&b in the Plateau area where you can usually find free on street parking, and you are close to all the interesting restaurants, shops and cafes on the Plateau. You can also find some very nice small hotels in Old Montreal, but prices will be on the higher end.

Hotel Le Dauphin.Parking other side of the street.Great place.Paul

Thanks, Myer, absolutely right.
Nearby is a little-known possibility, l'Hotel de l'Institut.
http://www.ithq.qc.ca/en/hotel/rooms/
This modern building, completely renovated in the last few years, houses the government's hospitality training school that includes a hotel and fancy restaurant, right next to the Metro stop. William-and-Kate joined a chefs' class there on their tour last year. http://www.ithq.qc.ca/en/hotel/rooms/
Rooms start at $129. 3535 rue Saint-Denis. The site says it has paid underground parking.

Also consider, in Old Montreal, a charming inn "Les Passants du Sans Soucy" www.lesanssoucy.com

we have stayed at Auberge de La Fontaine, a B & B in the Plateau neighborhood. It is written up in the Destinations section for Montreal. Lots of good restaurants in the area. There is usually street parking and the Auberge has a few spaces.

great mid-range hotel in a fantastic location: www.hotelsdauphin.ca