I've always been fond of the Plesiosaurus, ever since I was a kid. It's probably because it reminded me of pokemon (namely Lapras and Lugia) but it's large fins and huge, curvy neck was a thing of beauty to me. Of course, it's face is more terrifying than it should be for such a creature, but it makes it look more majestic to me.
I'm not exactly a dinosaur expert, but my top two have always been the ankylosaurus and triceratops! I had a small triceratops plush as a kid, that made I guess dinosaur noises, that I loved a lot.