European Hornets.
They typically build massive hives inside of dead trees. We had a hive so big it eventually killed half the tree.
I had them bad last year. They killed half my weaping willow tree and stung my dog a couple times. I used up 2 cans of spray, and have only seen the odd one on rare occasion since. I cut the dead limbs out and its starting to grow back this year.
Side note; wasp and hornet spray is a bi+ch to clean up off kitchen floors when you are trying to knock one down as it flies around inside your house.