Isn't the Irish Wolfhound the tallest breed of dog? They are wonderfully natured, but the breed is prone to significant health problems.
St. Bernard's, Newfoundlands, Great Pyrenesse (sp?), and Berneese Mountain Dogs are all smaller than Great Danes and Wolfhounds.
The most impressive dog I've ever seen are Leonbergers,
http://www.leonberger-dogs.com/
My family went to see a breeder in Wellington, OH - it was an elderly couple that had 7 or 8 of them. The first two they brought out were females, about the size of St. Bernards, and they were very nice, but a bit timid....then they brought a male out, it's name was Zeus - appropriate too, this thing was massive - it was about 220 pounds, not heavy looking or anything like that...but just massive...it's back was much wider than a foot, it was how I'd imagine a male lion looks in person - my kids were afraid, and it knocked my wife down just by bumping into her. If you are looking for a large breed, you have to see one of these!