Since no one website lists them all, I've compiled this for everyone's convenience. I couldn't find the date for the St. Lawrence Market competition, but given what I observed last year I wouldn't recommend them at all.
Definitely attend at least one of them, and do not pass up on Camp 31 as you will be missing the best. Other than driving out to Camp 31 in Paris or having them ship it to you, the Ribfest tour is the only place to get authentic Southern barbecue in the GTA. No Toronto restaurant can hold a candle to Camp 31.
Niagara Falls June 17-19
Toronto (Etobicoke) June 30-July 3
Timmins July 1-2
Markham July 15-17
Mississauga July 22-24
Scarborough/St.Catharines/London July 29-Aug 1
Brockville Aug 5-7
Guelph Aug 26-28
Burlington Sep 2-5
Oshawa Sept 10-12
I am not affiliated in any way with Rotary International nor with Camp 31 (or any other restaurant).