You can't beat late July/early August anywhere in the GTA...Scarborough Bluffs, Toronto Islands, Mimico, Port Credit, and Bronte .
From my experience, the south shore (Grimsby, St. Catharines, Niagara) is best late April to early June.
Smitty55 is right on...I have several buddies that move their charters from Port Dalhousie/Weller in the spring to Bronte/Credit for the summer.
That being said, anywhere on the lake all summer long will always produce some fish. Just keep in mind that often by late August/early September they start thinking more about spawning than eating.