The awe-inspiring Alligator Snapping turtle - they grow to a vast size (look the pics up of big ones on the web) and are truly prehistoric in appearance - a large individual can slice a broomstick in half with one snap of it's fearsome jaws.
A giant Brachyopoid Temnosponyl - not had much publicity but it could be the largest amphibian of all time overtaking Prionosuchus and Mastodonsaurus. It was absolutely massive and the head alone looks like it weighed well over a ton!