I call it the playground, although, technically, It doesn’t have a name because, technically, it doesn’t exist.
Coulson and team take over an old SSR base, which Eric Koenig calls the Playground. It was one of the first places he discovered inside Fury’s Toolbox, and became the main headquarters for SHIELD during seasons 2-4.
The Playground was a series of standing sets on the Culver Studios lot during seasons 2-4. It is never explicitly revealed where the location of this base is, but it is heavily implied that it is in the Maryland/Virginia/Washington DC area, due to the license plates on the SHIELD cars. Some have also implied that Delaware might be a possibility as well. An episode of the Slingshot webisodes suggests that Baltimore is a 20-minute Quinjet flight from the base.
(Agents of SHIELD S02E01, 2014)
MCU: Location Scout 