Olympic Cheese Mart is my favourite cheese shop in all of Toronto! You can find it on the top floor of the St.Lawrence Market right across from the Mustard Shop (which is also another must if you're in the Market).
Olympic Cheese sells a massive variety of cheeses... They have very friendly and knowledgeable staff (did I mention that there is even a cute cheese guy?), who will let you try any and all of the cheeses if you simply ask. My sweetheart and I have gone in a number of times and have asked for their recommendations regarding new cheeses to try based on the ones that we already like... And, they have recommended some real winners.
Our favourites are the raspberry beer or balsamic soaked Sartori, and the buffalo milk Parmesan. I also have a soft spot in my heart (and tummy) for the applewood smoked cheddar.
Olympic Cheese has undergone a recent renovation, and the shop is now very bright, clean and open (exactly what you'd want when shopping for food). The store not only sells cheese but also crackers, dips, etc.
The delicious cheeses aren't cheap... But the good stuff never is, right? Go spend your hard earned dollars on some Sartori... You won't regret it :)