Stegosaurus

Surely one of the most famous and recognisable dinosuars, stegasaurus roamed north america and europe in the late jurassic period.

Along its back and tail were large vertical plates which although fearsome looking, were unlikely to have been used for defensive means. Instead, they're more likely to have been used to catch the sun's heat and warm the dinosaur's blood. They may even have been used for display purposes, with changing blood flows creating varied colours to convey emotions.

As far as staving off attacks goes, that would more likely be dealt with by the spikes on the end of a stegasaurus tail. A sharp swing at a predator would be enough to deter all but the most fearsome attackers.