The White House and Capitol Building.
Especially with the Enclave Commander mod.
Nothing beats storming the Whitehouse with a battalion on your side. Especially when the Talon Company and Super Mutants are fighting. 3 way war FTW!
Calling in reserve troops, airstrikes, and other help, while your allies are fighting and dying...
You can even call in the Chimera Tanks from Operation Anchorage if you have the DLC. That thing, myself, and my troops vs. And army of Supermutants, a Behemoth, and a Talon Company group a is the most epic battle I have participated in.
It fucked my framerate to hell, but toning down the graphics and it still was far more epic than any battle in Lord of the Rings.
It needs to be on the higher difficulty to be as epic as described.
The trenches in from of the Washington Monument are also pretty epic.
I also liked The Republic of Dave. Mainly because its probably what I would do. Start my own republic with a few friends and name it after myself.
The place where the cannables live is also one of my favorites.
When you arrive its so ominous, the people there seem nice, but you feel like something more is going on...