i would have too go with raiden man.

a fucking deadly karate chop too the forehead would crack your entire spine directly in half at the power raidens got in his hand, not too mention he blows you a little lightning kiss as a parting gift at the end

Baraka or Smoke

Sindel's. It's the only one that made me cringe when I first saw it. It's horrific. I love it.

No one has mentioned...
1. Shang Tsung
Who knows what he does in that grab of his?
2. Cyber Sub-Zero
A big sword of ice showed through and up your body?

kitana...nuff said

My answer would be Sindel. I love every X-Ray move in the game, but most of them are actually fatalities and would you'd be dead by the last hit.

Sindel's is one of the only ones that would probably leave you alive, hence it's the most painful.

The kick to the groin just looks painful with her high heels, the scream to the face finishes it off. What completely puts her's over the top for me though, is when she breaks your kneecap. That just looks painful. She actually hits your kneecap so hard that she forces it backwards until it breaks.

That shit is just f'n brutal. Queen B is right.

After her I'd say Kabal. His is just painful looking as well. He snaps both your kneecaps. I still go with Sindel's though because it seems you'd probably go into shock after both knee caps get broken and then you get stabbed in the back with the hook swords. I don't think you'd be feeling much after that.

I think after Sindel's you'd still be conscious and in pain. The woman knows how much she can hurt you while still keeping you fully conscious and in pain.

Sindel.

From the top of my head:

Johnny's.
Reptile's.
Goro's.
Sindel's.
Jade's.
Kitana's.

Those (in no order except maybe for the first 3) are the most painful.

I'll post more later when they come to mind.

Also, worth to mention:

Baraka's.
Kano's.

Kenshi

Rain (Guessing)

I would sa Baraka walks this one, the pain from being impaled through the throat is unthinkable and to add to what would presumably be a long and clearly uncomfortable death, he then impales you through the eye and brain, which in most cases would cause an instant death including the split second of pain from having a blade inserted into your eye with much aggression. Lovely.
Mind you I wouldn't fancy my chances with any of them so it's hard to really determine a clear winner here, but if we were talking long term pain then any of the x-rays which crack the skull (Stryker, Raiden, Lui Kang etc.)

Sindel has my vote.

I'd say Kano. It's bad enough to get stabbed as well as snapping one of your bones, but imagine both stabs and snapping your femurs?

Melina, Kitana, Baraka, and Cyber Sub-Zero are instant death.

Kabal, Sindel, & Jonny Cage must be a hell of a pain..

Mileena: You wouldn't be killed instantly by the sais to the gut. You'd most likely be bleeding internally, with a broken jaw nonetheless after the second part of her X-Ray

Johnny Cage, Sindel and Kratos, I cringe everytime I see them.

as a male, im gonna say sindel or jade.

Kung Lao. Your head would be like a speed bag after that.

Shang Tsung. He crushes your lungs. Then he knees you in the face.

o____O

Just want to say that the audio department did a great job for Strykers X-ray. Specifically the part where he hits you with his baton Great sound effect!

Sindel for sure.

Because many are instant kills and lot of spine or neck damage witch would reduce you to a damn cripple.
But to think to recive a kick THERE, and then that knee break.... That is just damn painfull... And after these delicious stuff you'll get actually deaf, and may be confused and temporary blinded from that unimaginable head-ache.

It's my personal favourite X-ray.

Rain

A blinding flash that hurts your eyes.
Police flashlight on the top of your skull.
Baton at full force to the side of your skull.
A tazer right under your chin.

I heart you, Stryker.