Simple what's your favorite fallout expansion of all time.
Base your decision on
1) story: what dlc did you care most about the story line and which did you get most involved with
2) character: this goes from in game characters to the feel of the dlc as a whole (for example the Pitt- dark and scary or mothership zeta- shiny sify)
3) difficulty: which expansion presented you with a healthy challenge without it being to easy or impossibly hard
4)the loot: what dlc gave you the best loot from new guns and ammo to unique armour
Well that should cover the bases your turn.