It depends on what OP meant, as to whether it goes on how they're applied on screen or whether it's their real world application.
Del Rios cross arm bar(not arm breaker) is pitiful. In reality that is a devastating move. But as long as he has it on the elbow would be hyperextended and the oponents would not be back. Also all the sleeper holds are bad. You have 8 to 10 seconds before you are unconsious trust me I lost my first MMA fight to one. The spear joke how many running backs get hit and get right back up. Actually if you really thinkk about it finishers are supposed to be spectacular looking not realistic. We all love them truth be told. Although the HBK superkick would hurt like heck if he connected.
The arm bar is excruciatingly painful,
you will be in pain the instant it's locked in. As far as real world application, it's hard to find the right situation where you can successfully apply it but once you do, your opponent's fucked. Moves like the sleeper hold and the triangle choke as well are instant pressure. If you're in a triangle choke with full pressure for even a second or two, you'll be lightheaded.
The STF is the all time weakest finisher for me. Anyone who has done wrestling in a high school gym class can say that they have been in a hold more painful than that. The key to a submission is to have a pressure point and honestly I don't see anything with the STF. The leg isn't placed in an uncomfertable position and squeezing someones jaw is just uncomfertable, far from painful.
With the STF, the leg is locked, which decreases your opponent's mobility while they're essentially being choked. During the first few seconds of flailing around if they're unable to escape the pressure will rise and they'll be choked out.
I'm going with real world application minus the realistic element such as moving out of the way of the 5 knuckle shuffle. The Attitude Adjustment would hurt because they get some air when Cena tosses them, land awkwardly and you could screw your back and it'd definitely wind you regardless.
Also, moves such as the rock bottom and the bookend wouldn't be too painful. The upper back can take a pretty good pounding and you're not being slammed from a great distance, with that said there's also the risk of whipping the back of your head on the landing. So there's the risk in those moves.
For me it'd be a tie between
MVP's Playmaker and
The People's Elbow. The playmaker because unless you have impeccable balance, it'd be hard to get a decent amount of momentum to cause any serious damage. And the people's elbow because, at best, it might leave a bruise and you'd likely hurt your back more than you would their chest.