What's the best place in Montreal to meet new people ?

Well, you can meet nice people just about anywhere in any city, depending on the stuff you like to do. If you like skiing, head to the slopes. If you like the outdoors and its not to cold, try heading to Mont Royal. Every Sunday lots of people gather at the base of the mountain for a sort of hippy inspired gathering. If the bars are what you are into, try to head to some places on St. Laurent like Tokyo or Piz Pistol. If you head to Crescent Street, Sir Winston Churchill Pub is a good meeting place. Montreal is a city of lots of beautiful interesting people. They are all trying otbe found as much as you are trying to find them

Dépends on the season!  Un thé su,mer Time anywhere un Montreal like Hôtel Nelligan or  Place d'Armes or Quai des Eclusiers  are great places. In the winter time, I would go toward park avenue and Mont Royal and Boulevard Saint-Laurent. Great little places with that warm welcome quite needed during the winter months.

That really depends... Montrealers are by and large very friendly people and so, in principle, anywhere you go is a good place to meet people.  What style of bar/pub are you looking for (trendy, relaxed, ...)?  What language preference do you have (English vs. French)?  What area of town (down-town, old Montreal, Plateau...)?  FYI - Le Plateau is a cool area and not far from down-town.