Crit Chance and armor pen work well. Attack speed isn't too great as a rune in general unless it really synergizes with your champ (Nasus, Warwick passives). Crit Chance is less important, I'd have to say because of cleaver early game you're not going to be hitting any physical hits if they opponents know what mundo can do.
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I always wondered. Attack speed is good for him , so is armor pen, and so is crit chance when he crits he really does a lot of damage.