I would start Calgary to Edmonton. HSR between those two cities is a no brainer; you should be able to finish work, take a 30 minute ride to Calgary, have dinner, watch the Oilers destroy the Flames, take a 30 minute ride to Edmonton, and go to sleep in your own bed.
The best indicator of demand used in a lot of transport planning is the gravity model. Multiply the catchment population of the stations and divide by the square of the distance between the stations. Add it up for every station pair on a corridor and you get an estimate of demand. This model makes sense because the larger cities are and the closer they are, the more their populations interact.
Do this and you'll quickly find that the best city pair in the country is Ottawa-Montreal. Two large metros that could be commutable if there was high speed rail. The second stretch is probably Toronto-Kitchener-London. Toronto-Peterborough-Ottawa after that, especially if it's adding on to Ottawa-Montreal and/or Toronto-Kitchener-London. Calgary-Edmonton probably competes with Montreal-Trois Rivières-Quebec City for 4th place.
Also, Calgary-Red Deer-Edmonton in 30 mins for 300 km is impossible. Best case scenario is 1 hr from downtown Calgary to downtown Edmonton with en route stops at Red Deer and both major airports. The biggest beneficiary here would actually be Red Deer which would be 20-25 mins from each major airport and 30-40 mins to each major metro downtown. If this ever announced, I would expect Red Deer's growth to go bonkers.