I don't count dinosaurs in this. The T-Rex, Velociraptor, all those are high above this, I cannot not pick them, it is impossible.
But for the sake of the thread, I'll pick my second favourite, the Dunkleosteus, the first apex predator ever, living in the late Devonian era. Growing to a size of at least 10 meters (33 feet) and weighing at least 4.0 metric tons and being covered in armour plates made out of bone, this was one hell of a monster.
The whole Paleozoic era (that is, before the dinosaurs, starting with the Cambrian) is highly underrated when it comes to awesomeness. I mean, just look at the skull alone:
Move over sharks,
this is how it's done.