Footnote to the above post: It's actually top 7 firms since I have two tied at #4.

Interesting that Core is on the list. Some of their projects I think are fantastic, but most I find to be a bit boring.
The good: @12 Degrees, @M5V Condominiums, @Fashion House, @The King East
The boring: One Six Nine, Pantages, Players Club, @Seventy5 Portland, @Six 50 King West, @The Ninety, ZED, Zen Lofts
Note: I am not an architect, just going by what I see and like. I would love to recommend some architects, but I don't really know that much about them... So, without knowing who did them, here is my list of buildings that I think are really cool (built and unbuilt):
One St. Thomas (my favourite condo in the city), Domus, Exhibit. 12 Degrees, @Number One Bloor, @DNA 3, @minto775 King West. Also, I love the size and magnitude of @Aura at College Park!

Most architects are given specific design mandates by the developer for a project. There are some who flatly refuse
to design a building that doesn't fit their style. There are some who will do whatever it takes to win the contract.
Core is somewhere in the middle. They have root design principles which may be modified to accommodate the more mundane,
"safe" design requests by certain developers When they're not restrained, they produce some stunning work.
12 degrees and Fashion House are excellent examples of uninhibited Core architecture. I actually find 75 portland to be an excellent design as well.
Not a fan of Minto 775, DNA 3, or Aura but One St. Thomas is an AWESOME building!

Speaking only about exteriors, with no regard for interior space:
On the highrise side, I really dig a lot of the work by Hariri Pontarini Architects: The Florian, St. Thomas, Minto775, Five, One Bloor.
However, thanks to Symmetry opening my eyes, I have really been loving the lowrise and infill projects by RAW and Teeple. These guys are awesome, and have amazing styles.
Much like above, I am not an architect and I am not known to have the best taste.. so, take whatever I say as just my own personal humble opinion.
There are also some really great international designs I have seen, but as it is not my specialty, I would not dare try to figure out who the best are!