The Sharpshooter takes the win here due to it's popularity and because it's been used in some of WWE's biggest moments e.g Montreal Screwjob
But if you're basing it off a submission that looks legit, I'd go with the Regal Stretch. I believe it would genuinly hurt in real life, it puts massive torque in your neck, shoulder & arm but especially the neck since unlike most submissions, Regal stretches the neck sideways not backwards. It would be hard to break out of too. The Regal Stretch is so underrated.
I see people picking the Liontamer but I think that's unfair to the other submissions since that's rarely used in the WWE, so it doesn't really count as a WWE submission. It was mostly used in WCW but in the WWE, his main submission is the Walls Of Jericho, so it shouldn't count.