A lot of people will give different answers. Warhammer is a very character-driven universe. The lore is deep, but critical moments in the canon are made by certain heroes. I get the feeling GW probably wanted the "strength rating" of each hero sort of ambiguous to fuel fan speculation, but End Times was pretty exposing sans a few match ups that never came to be.
People will be quick to say Aenarion, Grombrindal, or maybe even Vlad Von Carstein -- there are some more "minor" characters which are known for their prowess in dueling. Wulfric The Wanderer jumps to mind, as well as Malus Darkblade. Take your pick really!
As far as most powerful mages - Nagash (although a sort of "ancient" character) is widely considered to be one of the most powerful, if not the most powerful magic wielder in Warhammer... maybe second to Kroak. I don't care much for the mage character so someone else can chime in.