Which Keystone to take on Mundo?

I would pick Grasp of the Undying.

iits all contextual actually MY personal favorite on him is Grasp of the undying. (Heal while smashing peoples face in!! SO FUN!!!) for jungling tho, i take the damage reduction one that gets stronger when i gank (The reduction is actually VERY helpfull for early game, and makes you have more health for ganks. Plus failed ganks wont hurt your laner as much (Although if you have more than 15% health it WILL hurt YOU.) and the other one is actually not that strong on him, only certain champs benefit from it, and mundo isnt one of them (Extra health? sure, but you lose battle capabilities.) Also, its dependent on the enemy team, if they are poke based, i take the reduction one no matter where i am, because most of the time i wont get close for the heal (And im a pretty agresive mundo player, but i prefer safety over kills). BUT if they are all in teams and close range heat of battle gods (Jax, Fiora, Yasuo, Talon) I take the heal while i battle.