There are five Super Mutant Behemoths that can be found and killed in the following locations. Note: Once they are killed they do not respawn:
Galaxy News Radio This Behemoth will destroy a wall just outside the GNR Building as you enter the area (quest related).
Evergreen Mills At the bandit camp near Vault 112, a Behemoth is behind an electric enclosure beside the train tracks. When the generator powering the enclosure is destroyed the Behemoth will escape.
The Capitol Building Use the west entrance and go inside the main room. Look on the map. The room is straight down the hall from the entrance. However debris will force you to find an alternate route there.
Takoma Industrial Located east of the GNR Building at the end of the map. The Behemoth is just north of the factory, standing in the middle of some wrecked cars. Jury Station Go west until you find a group of crumpled up train cars. Search this area until the Behemoth appears. You will know if you are in the correct area if you see a teddy bear in a shopping cart.