Continuing with the favorite monster theme, for this category I'm going to choose the fire elemental. Yeah, it's simple, and it's not a big bad villain-type monster this time, but I love the simplicity of the fire elemental. It burns, and that's it's big attraction. It's an uncontrolled blaze that managed to get itself summoned to the material plane (at least that's how it usually works).
It's possible to use magic to control them, but the interesting part is what happens when the magic fails. What happens if a wizard loses control of his fire elemental in the middle of his laboratory? All the arcane stuff in there is bound to have some sort of effect on the elemental. Or what happens when a fire elemental gets loose within city limits? The townsfolk generally can't hope to contain the blaze – that's where the PCs come in.
It's a simple monster, but it can lead to bigger situations than just “okay, there it is, let's kill it now.” That's where the draw of the fire elemental is for me.