I'm glad to see a thread like this hovering near the top 'cause Monsters are primarily what I am having difficulty with in one of my favorite fantasy WIPs...
I think my preference would be towards new monsters (i.e., not stolen, except perhaps adapted, from folklore), who are sapient enough to have motive beyond just "need eat: must eat". That being said, I am apparently incapable of achieving that standard myself lol.
Granted, I think some of my preferences stems from what I was trying to create, which was a certain hierarchy of monsters. The basic monsters I might be content with either pulling from common folklore or mutating a species of animal into a monstrous form, but the advanced monsters running the show... I haven't quite figured out what to do for those.
I think my final opinion may be summed by (perhaps unsatisfactorily): it depends on what you're using the monsters for. As an element of environmental danger or as a serious antagonist. Maybe I'm thinking too narrowly? I don't have a lot of experience with monsters.