From what I understand about dinosaurs, the T-Rex had a very well developed strategy center of the brain (hell the brain itself was colossal too) and the power behind it's jaws was practically overkill most of the time. T-Rex's more difficult prey included the "living tanks", Triceratops and Ankylosarus, and I say "more difficult prey" because there was really no reliable T-Rex counter, they didn't hunt them for eating unless they were desperate. T-Rex was smart and strong, so it knew how to exploit these guys weaknesses and destroy them with one well placed bite, as evidenced by fossils of it's victims. So I'm going with the T-Rex for best dinosaur, who practically ran that Late Cretaceous shit with both hands tied behind it's back because it couldn't use them anyways. I guess they moved pretty goddamn slow though and didn't take falling down well at all.
Some say a particular kind of Raptor could have been the best dinosaur if they had time to develop wings they could fly with and use to get the drop on unsuspecting prey, as they were also very intelligent, highly social and were armed with a plethora of sharp things. They were pretty small for dinosaurs, but they were excellent hunters and were fast enough to escape from dinosaurs they knew better than to try to fight. Pretty damn good all around.