In my opinion, Operation Anchorage is a good one to get because it gives you plenty of good stuff for use in the wasteland when you return.
Broken Steel was probably one of the best, because it lets you continue the game after the normal end, and it gives you more quests then too.
Mothership Zeta also gives you a lot of good stuff, and it's quite fun too. You can also return to the ship after you're done with it.
The Pitt rewards you with plenty of stuff too, but it's not as worth it if you aren't willing to search for all 100 of the ingots. Not sure if you can return here.
Point Lookout was probably the most fun for me. It has a nice storyline to it, and plenty of quests. You can also return to it after you finish.