Shen or Renekton?

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Shen's E has more lockdown than Renek's W.

If it hits, and it's a lot less reliable.

Shen is much better waiting for people to dive onto the carry and then peeling them off.

True, but Shen really becomes a one ability and ult champion in lategame - only your E and R are really meaningful. W's ok but generally too hard to time for it's cooldown.

Renekton's mobility is fairly bad. In a teamfight, any champion with cc will stop you, and then you will get melted since Renekton really is not that tanky lategame.

Too long range dashes, and any CC used on him is less for anyone else.

Meanwhile no one will ever use more than a single CC ability on Shen because they simply don't have to - he's not fast enough to require CC to lock down, he's not enough of a damage threat to be worth CC'ing to protect your carries. Any meaningful CC on him will be to stop his E, making Shen into a juggernaut without damage.

His stun only lasts for 1.5 seconds. Very mediocre cc. And that is at 50 fury. If you are dashing through two targets to reach the ADC, then you will likely use up your fury unless it is at 100%. Then the stun is only for 0.75 seconds. That is, if you get to the ADC, because the ADC alone can probably just kite you and blow you up, especially if its a Caitlyn with bloodthirster.

implying Shen's mobility is in any way better

The only way Renekton is better than Shen late game is if Renekton snowballed the early and mid game and is now super fed. In that case, the ADC is probably behind so you can just oneshot them since their damage will literally tickle you. Once an ADC starts getting some items, Renek just turns to poop. At least Shen has the auto attack nullifier on his W and gives his teammates shields and cc.

forgetting that Ren has a 750 health boost on his R, more than making up for Shen's passive shield and W. Only when it comes to shielding allies does Ren fall short.