well, pre-expansion and among level 60 staffs it would have to be Atiesh, Greatstaff of the Guardian. Level 60 Legendary item, made from 40 rare drop splinters that drop off Naxxramas trash and bosses, the base that drops of C'thun and the head that drops off Kel'Thuzad.
So it more or less takes a raid of 40 people and a common goal to assemble ONE of these staffs.
There are 4 versions of the staff, one for each mages, druids, priests, and warlocks.
More info on Atiesh can be found here
Link