I agree with VIR being great but so is Laguna Seca! Those two would be top American tracks, but I agree with Gleb, that Mosport GP is the best in N. America!
I am also an instructor and V.P. with Trillium. I'll try to not be biased though. Ottawa runs a tight ship as does Trillium, we have the same curriculum, and some of the same instructors. So, quality will be top notch either way. Trillium runs 3 - 3 day events at Mosport. If you have the budget I would suggest you attend both schools to get different tracks under your belt. I do personally prefer Mosport, but Calabogie is also awesome, a very close 2nd. Mosport more wide open with faster sweepers and straights, Calabogie more turns and more technical.
Hope this helps......
BMW 325I E30 Chumpcar Race Car
BMW 540iT M-Sport (New Family Hauler)
BMW 135i Msport Daily/track toy