Just by how often I’ve used them, I’ve probably got to go with demons, but I also like using kobolds. Nobody expects kobolds to be anything more than a low-level encounter, worth no XP, but there are lots of ways to use kobolds that can make them challenging if you know what you’re doing. Traps, dire weasel pets, archers and sorcerer leaders, plan it right, and you can have a lot of fun with them.
For me it’s almost evenly split between undead and human/demihuman.
Most of my campaigns have had humans or demihumans as the opposing force to the players, whether as overarching villain or single-adventure threat. One of my favourite aspects of D&D is crafting unique, complex NPC’s for the players to interact with.
But undead edge it out slightly. I don’t play them as often, but I love the oppressive, hopeless atmosphere of a kingdom ruled by vampires, or a world populated by zombies, where the players have no assistance outside their own ability.
I voted Demons, but I have equally used Devils. Its awesome to have a PC actually make a deal with a devil. I had one such PC, where I had actually typed the contract and printed it. It got all “Willy Wonka"-y and small towards the end. Ah, memories....
I did dragons. Although the reason I picked them could be used as an argument for many other monsters.
I like my villains to take a very long term look at their plan. And you need something that lives a long time to have their plans come to fruition. So yeah, undead, demons, devils, and the like can all fall into that same category, but I just like dragons.
My group recently stated I have a love of undead, golems and certain types of demons, which all share a particularly annoying trait for them of being immune to crits. ^^
But undead is where I get most of my mob fun out of. Closely followed by demons.
They may make use of monsters or undead, but the BBEG is usually Human/Demi-Human. I’m not above bringing them back from the dead, or making them undead for repeat performances. One the PCs actually resurrected a repeat offender after he was banished to Hell. That time he came back with diabolic powers and an ancient blue dragon.
Who in their right minds uses a resurrect on an blacken skull found on an evil altar in the ruins of an arch-nemesis? When asked why he said it seemed like a fun thing to do at the time.
