Unfortunately the places with the best food don't have the best ambiance. If you want good ambiance, Gilaneh in my opinion is pretty good.
Shater Abbas is very good food, but the restaurant decor itself is very ordinary.
There's also some restaurants which the hipster crowd love such as takht tavous and pomegranate but they're marketed towards non-Iranians.
There is a very good restaurant called Reihoon in Burlington (near Hamilton), pricy but high quality and great setting.