Louis' is one of those restaurants in Ottawa that has a bit of a cult status with residents, but its existence isn't usually known by visitors or newcomers. If you asked many residents in a poll what the best pizza place is, many of them would say Louis'. I don't necessarily agree (Colonnade all the way!), but it's definitely up there on my list.
Their pizza is delicious, simple, extra cheesy and uniquely flavoured. I think they use nutmeg and/or cinnamon in their dough because it has a very mild spicy sweet flavour usually found in cinnamon buns in stronger doses.
The shop looks more like an old diner than a pizzeria; they even have a jukebox, which just adds to the flavour of this establishment. The servers are very down to earth and they often seem to know a significant chunk of customers in the room, since Louis' definitely has its share of regulars.
Also, if you're lame like me, go to the bowling alley behind Louis' for a fun evening with friends.