Thanks for the great question. It's one I have thought alot about.
As a kid, my favorite dinosaur was, of course, the Tyrannosaurus Rex. In fact, I had a beloved plastic Tyrannosaurus toy who aided the Rebel Alliance in their battle against the Galactic Empire and Skeletor.
But as I grew older, I fell out of love with this famous dinosaur. The Tyrannosaurus Rex is, basically, one of the most over-hyped great lizards of all time. Just look at the giant galoot. How could he have been the most fearsome predator of all time? For instance, most great predators have strong forearms for holding down meat that is still fighting back. This makes it easier to tear off chunks of flesh. The T-rex has those spindly little doll arms. And there is no way the T-rex could run as fast as a jeep, as demonstrated in the totally fake documentary Jurassic Park. While it had impressively large legs, the T-rex was top heavy. Most likely, the T-rex was just a common scavenger that would lumber around, using it's back legs like a seesaws fulcrum, rising to spy for carrion, and dipping to take bites out of any it discovers.
Lame.
I would not want to be a dinosaur that is all hype. One dinosaur who lives up to it's reputation is the triceratops -- and that's the dinosaur I'd most want to ride into battle if I was ever transported back in time to a land of prehistoric savagery. The horns, the shield, and the ease in which it would wear a saddle all factor into this scenario. But that's another topic entirely. Oh, also, velociraptors? They're like the rat-ostriches of the dinosaur world. Totally overrated.
If I had to be a dinosaur, I'd either be a pterodactyl or a stegosaurus. The pterodactyl because it can fly. A flying dinosaur is badass. The stegosaurus is a huge beast with intimidating spikes on his back and spikes on it's tail. It's a truly dangerous looking creature. But it was a plant eater. I like the idea of being a dinosaur that is strong and fierce on the outside, but a peaceful salad lover on the inside.
The whole T-rex predator vs scavenger is still up for debate. It was most likely an opportunistic carnivore - eating slowing moving live prey but also surviving on carrion.
Pterodactyl was not a dinosaur. It was a flying reptile. Get it right.
Also - it was recently discovered that Triceratops are actually just baby Torosaurus. But since the Triceratops was named first the Torosaurus now takes the name of Triceratops. So, Yes there are triceratops - but they are even cooler than they were before.
But you're right about Stegosaurus: pretty awesome.