What are Dragons most vulnerable to?

Dragons are vulnerable to fire: well, at least some of them.

There are many types of dragons, but they fall into two major archetypes: fire dragons and frost dragons.

• Fire dragons are vulnerable to frost (25% weakness) but resistant to fire (50% resistance).
• Frost dragons are vulnerable to fire (25% weakness) but resistant to frost (50% resistance).

So if you're trying fire and it's not doing a lot of damage, switch to frost. But you should be able to tell what type of dragon it is by the type of breath attack it has.

Yes, all dragons can be hurt by fire. After all, if think about it, they are magical creatures and well, it's destructive magic.

Not to mention, some dragons can be more hurt by fire than others. It's just common sense that a dragon with ice breath wouldn't like fire, whereas a fire dragon would dislike the cold.

So the above answer is pretty good and does outline which magics work hurt what types of dragons.

However, there is a second part to this question that I don't feel got answered.

"What magic is most effective against dragons?"

While fire and ice magic does have hit power, it's not always the best approach. Dragons can show up without notice, and skyrim has a diverse climate. There are times where both a fire and ice dragon can appear.

At such times, it's not always practical to cast the typical fire/ice magic.

This is why I go for shock magic. It works the same against each type, allowing me to more fully prepare for sudden attacks. I like to temper this with healing magic, so that I don't have to pop potions every time I get low.

That way, I'm self sustaining. Dragon shows up? I'm ready. Even if my magic regen is low, as long as I can get cover, I can keep healing. It works wonders, even at lower levels.

The other answers are all great, but it still takes too long to kill them that way. What I use is the first level Marked for Death shout found in the dark brotherhood sancurary. Since its recharge time is short, you can use it 3-4 times and their heal drop by a lot. Even works on Elder and Ancient dragons. By using a 105 bow, I was only able to take out 1mm of health, but using the shout a few times, my bow can take out an ancient dragon with only 5-6+ hits with a 105 ebony bow.