I was wondering what is the best order to do the addons and at what level?

Depends on how you want to play, I always play stealth based characters so I always do Operation Anchorage for the Chinese Stealth Armor first. Then since I like energy weapons I normally make my way to Mothership Zeta, it's something sixy about a riffle with 100 shots per reload with a 1 second reload time. And it hits, hard. Other than that I make my way to the rest of the DLC whenever.

Operation anchorage - lv 7

The pitt - lv 11

point lookout - lv 14

Mothership zeta - lv 18

How I did it:

Operation Anchorage (Sneak is pretty much ONLY usefull with the Chinese Stealth Armor. I'd do it A.S.A.P)

Mothership Zeta (which I only suffered through for Paulson's Revolver and Samurai Armor)

Point Lookout

Broken Steel (I like this one most)

The Pitt