Having played the full game and for 100+ hours, it's mostly Drowners, Ghouls, Wolves, and Bandits. Drowners may actually be the most common monster in the game, but I could be mistaken. I really don't even pay attention to what comes up anymore, I won't lie.
Drowners are grossly overused, and reaching Skellige makes you realize the game has a complete and utter lack of aquatic enemies in general. I'd be glad to see 30~50% of the Drowner locations mixed with some other appropriate monster, and have entirely new aquatic monsters, period. Same goes for Sirens/Ekhidnas (Another Siren) in Skellige.
Also, people need to stop being "that guy". No, just because somebody complains about a lack of or overly used enemy types doesn't mean they expect a unique enemy around every turn. That kind of response is incredibly stupid. That's the kind of response a child would give somebody for crying out loud.