This century, none at all.

Last century, there are potentially two:

Nuclear - The Tsar Bomba, theorized to be 100+ megaton, tested at 57 megaton.

Conventional - The MOAB (Massive Ordnance Air Burst, or Mother of all bombs), GBU-43/B, thermobaric device.

The GBU-43 (Massive Ordnance Air Blast, usually called a MOAB. Mother Of All Bombs) which is a 21,000 pound bomb with a blast yield of 11 tons. The U.S. Air Force Research Laboratory has stated that a larger version of the MOAB exists, weighing thirteen tons.

Impossible to say- there have been only a very small number produced, and those came from the budget that was funded to develop the MOAB. The guidance package used in the MOAB is about $20,000 each, plus the cost of the MOAB itself.

Yes, the B-52 can carry the MOAB (Massive Ordnance Air Blast) bomb. However, it typically requires modifications in order to carry and deploy the bomb. The B-52 has sufficient payload capacity to carry the MOAB, which is one of the largest conventional bombs in the U.S. military's inventory.

An air burst is the explosion of an air bomb in the air rather than on the ground.