What skill gives the greatest knockback for a wizard?
Your best bet is Wave of Force with the Impactful Wave rune.
Wizards don't have a skill that will reliably cause Knockback. If you're finding that enemies are closing on you quickly, try dropping a Slow Time with Miasma or Blizzard with Grasping Chill or Frost Nova with Shatter. These aren't Knockback skills, but they do slow/freeze very well, allowing you to flee or attack with impunity (for a second or two).