There is always this item, that never dropps, that no matter how many runs you did, you lost to rolls or not enough dkp.
For 3 years on retail servers, i never ninjaed anything, but i was there so many times, when someone did this, they have even ninjaed a few items on tbc and wotlk , that I desired the most.
IMO ninja-loot SHOULD be bannable - a day, a week, a month than permabann, yeah this will be great.
And back to the question - what would you ninja, if you are master looter and theoretically you can?
My choise is : Tempest of Chaos
http://www.wowhead.com/item=30910
dropped 2 times to me, 1 lost to dkp, 1 ninjaed by ML)