What was the nicest dinosaur?

None of them would likely have made good pets. Though highly advanced for reptiles, dinosausr still didn't have the kind of brains that make for affectionate behaviour. Herpetophiles (people who have reptile pets) say that alligators are "sweeter" than crocodiles, but by that they just mean less aggressive.

There's evidence that at least some dinosaurs cared for their young.