i would definitely bring back the Megaladon. The only good sea monster we have today is the giant squid. Although it's awesome we need sea monsters. Well, we don't need them but I have a special place in my heart for sea monsters and the mythology surrounding them.
I'd pick Baluchitherium. They're so freaking big: It'd be cool to have some walking around. As long as it's not somewhere close to a big city or something.