Is it legal and safe to have an electrical breaker panel installed outside of an external wall of a single family home in Ontario Canada?
There are plenty of panels which are UL listed for outdoor use; that's what NEMA 3R is.  

However, the outdoor environment is tougher on panel components.  For instance GFCI and AFCI breakers will have a much shorter service life outside.   The threats are dust and moisture (mostly via condensation). 

It has been suggested to me that placing a rather small heater inside a panel can help drive off condensation, but of course will cost a small amount of money (typically $1 per watt per year) and will alarm anyone who looks at your panel with an infrared camera (normally warm panels are a BAD thing). 

One option is to put the main breaker outside and the rest of the service panel(s) inside.  

If this is part of a recent trend to eradicate all utility spaces from a home, I think that trend will backfire.  Houses have utilities (unless you're Amish).  Utilities need maintenance.