I know that cottage-bound escapees from Hogtown often stop by Weber's haburger stand on a few miles North of Orillia. These are excellent hamburgers.
However, if you happen to drive through Newmarket along Mullock Drive (first Newmarket exit when heading North on 404 - and a good alternative to the 401 and 407 if you need to stop / shop for anything on your way up), there is a small mall with a Tim Hortons and a supermarket.
Tucked in the far corner is Biggie Burgers. This is purely a one-off. As far as I can tell, it is part of no chain. Whenever I've eaten there, the owner (I presume) is dutifully hamburgers and other fare - fresh. There are but three or four tables - and a bench for seating.
For a small establishment, the menu is quite extensive - so the non-burger consumers are not left hungry. On my last visit, one of the staff explained that they take all of the pictures of the food themselves.
Anyway, it's more than worth a stop - great food made with care, and clean and friendly.