Who is the best wizard

Game wise Teclis may be up there, but fluffwise I'd have to say he wouldn't be a major contender to Kairos or Kroak. Nagash would be up there as well, he basically obliterated an entire nation with one spell...

Teclic would be the most powerful 'living' Wizard though, as the other three are all effectively all spirits.

The lastest WD basically names Kairos.

"There is one being in creation that has mastered the wheel of magic and can use all eight Lores"

Considering from the fluff it is entirely possible that he could use High, Dark, Wild, Athel Loren, Necromancy and all the other Lores as well.