I like the Troodon as well but I would pick the Ankylosaurus. It's a dinosaur that doesn't get enough attention. Also it has always been my favorite it's got armor, spikes and a blunt club tail. Ankylosaurus is a dinosaur that could take a beating and come out of a fight ok.
Ankylosaurus is a survivor there were a lot of things that could hurt that dinosaur. It's tough, rough, strong and hard. Ankylosaurus is not known for size and speed but it was a dinosaur that you wouldn't want to fight. I always pictured Ankylosaurus as an easy going dinosaur but if you start a fight with one lot out. It's armor and spikes could make almost any creature back down and deflect almost anything.