Might need to buy the GOTY edition if the DLCs aren't available individually. If they are I still recommend getting them all. But the priorities are Broken Steel, Point Lookout, and The Pitt, in no particular order. Broken Steel arguably adds the most to the game because it makes the game continue once the main story is complete like FO4, if that's important to you. Point Lookout has the biggest sandbox addition adding a good bit of unique content with a good story. And the Pitt has arguably the best story with a compelling decision you must make. Mothership Zeta and Operation Anchorage are fun but not on the same level as the other three. However, they both give some really good loot.
