I always go with the classics...
- wild animals (wolves, bears, tigers, giant rats, giant snakes, giant spiders... depends on the environment)
- bandits/pirates (greedy but not suicidal), goblins (coward and mischievous), orcs (brutal and chaotic), hobgoblins (expert and regimented)
- skeletons and zombies (particularly when I don't want to have to explain the ecology of the locale)
- giants and ogres (as solos at low levels)
In the past I have been particularly fond of using undead, but they have become so ubiquitous that I feel I would like to remove them from the stories for a few years, in the attempt at making them feel more "scary" again. I mean, if I would ever meet an ogre or other monster IRL I would be scared, but if I would ever meet something that should be dead and instead it's not... I would be a lot more scared! They are just
wrong. But they have been so overused in fantasy by now, that people just don't even think about the issue anymore

.