Whenever I'm creating a dungeon, I always have to ask myself how monsters I place survive. What do they eat? What do they do all day? Do they just stand in a room waiting for the PCs to come along? Undead are the best for this reason. Standing in torpor actually makes sense for them. I don't need to worry how they survive. They can be mindless or they have ancient machinations that they've continued to advance over the ages and anything in-between.. They're the perfect dungeon monster.
Aberration 22
Beast 18
Celestial 19
Construct 20
Dragon 32
Elemental 20
Fey 23
Fiend 14
Giant 20
Monstrosity 25
Ooze 10
Swarm 9
Undead 13