5,6%
http://www.wowwiki.com/Defense
When an equal level mob is attacking a player, the mob has a 5% chance to critically strike the player. For every level beyond the player, there is an additional 0.2% chance for the mob to critically strike the player. Therefore a level 83 mob (a raid boss) attacking a level 80 player has a 5% + (3 x 0.2%) = 5.6% chance to critically strike the player.