This post is very similar to my earlier post about my favorite elemental monster, and the answer is pretty much the same: fire.
It's the classic dragon breath weapon. It's a key part of one of D&D's most iconic spells, Fireball. It's often the byproduct of a wizard gone amok.
Fire is interesting because it can take so many forms during an adventure. It can be a living threat in the form of a fire elemental. It can be a part of multiple spells (think pyromancer). It's a red dragon's signature weapon. But, it's also an event. A village burning down, for instance.
It's also a good thing for adventurers, though. Most parties wouldn't get very far without a healthy supply of torches, after all, nor would they even get to most dungeons without camping in the wilderness and using fire to light up the darkness and keep them safe at night.
Fire is both helpful and potentially devastating to adventurers. That's what makes it so interesting.