Assault armor is very good mitigation for the price, or riot if you get hit with melee a lot but warlord is only on guns so if you war assualt is usually better. Just the assualt helmet is crap so I'd recommend a riot helmet for either option. You lose the set bonus but get full coverage and block pepper spray.
If you want to spend billions the 4 orange sets are nice depending on play style. One for defense and one for dexterity stat boosts. Then there is the one that reduces the strength of status effects and another that increases max life.
Then there is the red set that has a chance to mitigate 100% of damage but is 10s of billions.
Look in the auction house or the wiki.