The two you should get -
Broken Steel - Allows you to play past the end of the main quest, raises the level cap by 10 to 30. Add snew perks, quests and Dog Meat becomes strong so the sadness of his death is avoided. This one is a must have.
Pointlookout - New landmass to explore which is very big, new weapons, quests, loot, cool characters etc. If you are going to get any extra to compliment Broken Steel, this should be it.
The other two are too short really.