While I agree with the other person that you should not limit your scope to the GTA, there are some really good schools around the traps.
Spectrum Airways which is between Milton and Burlington. Go talk to their CFI, who I think is still Audra Oakes. She has an excellent staff and is one of the best and most patient instructors I have ever come across! They have a decently maintained fleet with very few disruptions, and as it goes are probably the most ideally located school in the GTA with its proximity to uncontrolled and controlled airports, instrument approaches for the instrument rating, and is only 45 minutes from Niagara Falls and the US border (good for sightseeing and trying out the American way of doing things once the licence is done!). Their website is:
www.spectrumairways.com
I have never heard a bad thing about Brampton Flying Club either, and know two ex instructors who are nothing but some of the most professional people I have the pleasure of knowing. There is also Guelph. Kitchner/Waterloo, Toronto Airways and Im sure there has got to be one out in Buttonville.
The only problem with training right in Toronto is the proximity to Pearson, which at times will limit when and where you can fly, making ab initio training a little more difficult than it needs to be!
Hope this helps!