Allosaurus was my childhood favourite, because there was a cool picture in a book we had. But like many others, I jumped on the Velociraptor bandwagon after Jurassic Park, because they reminded me of Jack Russells. Triceratops are ace too, because they can take on a T Rex..
Stegasaurus and T-Rex. If only for the interesting fact that there was a longer timespan between Stegasaurus and T-Rex, than there was between T-Rex and the iPhone.