I think the Diamond Cutter aka RKO is the best finishing move ever. The stunner needs a kick setup and but the RKO can be done from all different angles and out of no where. A close second I would have to say the Crossface. That move is very affective and it seems like everyone and anyone taps out to it. I love the was Benoit grabs the arm in different ways to get it locked in.
Out of the list I would chose the Sharpshooter, just because it's a submission and painful looking thing but there's always a chance the person can break out.
But UT would get my vote with the tombstone or last ride if it was there.
When you see Jerusalem being surrounded by armies, you will know that its desolation is near. Then let those who are in Judea flee to the mountains, let those in the country not enter the city. For this is the time of punishment in fullfillment of all that has been written. How dreadful it will be in those days for pregnant women and nursing mothers! There will be great distress in the land and wrath against this people. They will fall by the sword and be taken as prisoners to all nations. Jerusalem will be trampled on by the Gentiles until the times of the Gentiles are fulfilled.
...At that time they will see the Son of Man coming in a cloud with power and great glory. When these things begin to take place, stand up and lift up your heads, because your redemption is drawing near.
Luke 21:20-28
The Stunner for pure speed and visual impact, and the Tombstone Piledriver for devastation and real impact. The idea of a delayed piledriver that lets all the blood rush to the head before impact so as to completely disorient the opponent and compact his spine/neck at the same time is genius. Of course it also fits the Undertaker gimmick really well too.
If we arent limiting ourselves to just WWE or the listed finishers, I really like AJ Styles' Styles Clash. Easy setup, effective anywhere, its wicked.
Jerry Lynns Cradle Piledriver also looks brilliant visually (the added downward thrust of the arm keylock on the leg makes it look that much more vicious). Theres also another by a guy called Jody Fleisch that competes in a promotion called FWA in England, called the 720DDT, and its basically a twisting springboard DDT (he does a 720 degree revolution while hitting it). Visually stunning.
Outside of that List I would certainly have to say Victoria's 'Widows Peak' or Reno's 'Roll of the Dice'.
Confined to the list though, I would have to say The Stunner.. Mainly because its one of the few protected moves in the WWF and not many people kick out of it.