There are certainly better watch dog breeds if this is primarily what you are looking for, but if you are looking for a great companion first, that can also function as a watchdog, then the Frenchie is a great choice.  You can count on your Frenchie letting you know when a stranger comes to the door, or the house is on fire.  Once they are confident that there is no imminent danger, they will return to being the sweet, calm, friendly dog they are.  Strangers do not stay strangers for long :-).