Which dinosaur would have been the best pet?

I'm sorry, but none.

If you had a Carnivore on a leash or in a cage, it would feel restrained, and try to break out, which it will probably succeed to do, and if an angry Carnivore was on the loose, things would not go well.

On the other hand, if you had a Herbivore, you would have to feed it it's own body weight a day, and for a triceratops, that would be 4 to 6 5 tons!

I don't think any dinosaur would be a good pet.