The Dunn's in
Ottawa is on Elgin Street, between Cooper and Lisgar.
Whatever its flaws, it's one of the only 24-hour joints in town, and therefore one of the best. When you're drunk, it doesn't matter as much. The club sandwiches are the best deals.
The food isn't as good as at the Elgin Street Diner, another 24-hour-joint down the street, but it's cheaper. The service isn't as graceful, but it's quicker. The décor isn't as good, but ... well, the décor isn't as good.
The one you prefer depends on your priorities. The Elgin Street Diner's breakfasts are undeniably superior, though.
The Ottawa Dunn's has two very different clienteles. Late at night, it's the standard complement of grad-student couples, drunks, lonely folk, and teenagers up past their bed-times. During the day, because it's nearly across the street from City Hall and the regional courthouse, it's packed with politicians, bureaucrats, and lawyers.