I've been to 3 places. Depends on the atmsphere you are looking for...
At the Chateau Laurier in Zoes Lounge - surrounded by windows overlooking Wellington St. served daily - silverware and china with 3 tiered plates - large tea caddy is wheeled around and you can select your loose leaf tea. 3 types to choose from - Traditional Afternoon Tea, Canadian High Tea or Champagne Tea. Full menu and prices which range from $25 to $50 can be found here http://www.fairmont.com/laurier/GuestServices/Restaurants/ZoesLounge.htm
The more affordable option is Tea Party ($10) - artsy atmosphere - huge selection of loose leaf tea. The food is fresh and tasty and large portions. Strangely the scone has to be ordered as well as the afternoon tea.
15min drive from downtown, there is also the Moorside Tearoom at the Mackenzie King Estate - can also enjoy on terrasse in beautiful surroundings of the Gatineau Park, food is okay (serve the traditional scones, clotted cream and jam, sandwiches, tartlets - but it's being able to walk around the wooded area, trails and lovely gardens and getting fresh air before sitting down that I like. The Estate was the summer residence Canada's 10th prime minister.
Please report back as to where you end up