Whether it's the best finisher or not, it's probably the most over.
Which more or less equates to it being the best, so yeah.
I think a large part of the appeal of the best finishers are related to a small number of criteria, which if you look at the most over finshers, they all share similar structures. Here are the things which I believe, make the best finishers the best:
1) Short with next to no set up required
The move can be hit unexpectedly, which often produces a far more excited reaction than a move with a longer setup. Although, sometimes longer setups build more tension and also lead to a great reaction. See Rock Bottom vs. People's Elbow, or Randy's "The viper is coiled!" RKOs vs. his "Outta Nowhere!" RKOs.
2) Can be performed from a multitude of positions
Because it adds even more 'Outta Nowhere!" factor. RKO, Orton's learned to counter aerial moves into that shit. Which frankly, is awesome. This alone puts it right up there with the best finishers. Also, he can do it from the mat or standing.
3) Relatively simple follow-through from a recognisable clutch
Yes, we love Sin Cara's crazy tilt-a-whirl headscissor whatever, but the best finishers are the most easily recognisable and simple maneuvers. Compare Sin Cara with Evan Bourne. Bourne has a better finisher. Shooting Star Press is rarely done and although difficult to actually perform, is pretty simple in terms of its shape. Again, see Stunner, AA, Rock Bottom, RKO.
Having a recognisable clutch to start the move, which is not frequently used in other moves very much, also helps. See the double underhook to start the Pedigree. I also think this gave McCool one of the most recognisable Diva finishers, in spite of it being a Styles move. How many other moves in WWE start with a double underhook these days? The Future Shock, and that's probably it.
4) Looks like it makes an impact/inflicts strain
The 'strain' is for submission finishers really. This is obvious, but important. A finisher needs to look painful.
5) NAME OF POWER!
I'm gonna use newcomer Jinder Mahal here. His finisher is a Full Nelson slam - Excellent choice, it's not often done in WWE, it's short, easily recognisable, impactive, and people were already reacting to it by like his third squash match. However, Josh and Cole keep saying 'Oh, here's that maneuver again!" Which needs to change because it can't go on being 'that maneuver' forever.
Names of Power should either help a wrestler's gimmick, or enforce that the wrestler has worked on making his version of the move more devastating. Remember, Alberto doesn't do an armbar, he does an 'armbreaker'. If not that, then the name should at least make the move stand out. A lot of people don't like Wade's Wasteland, but at least it has a much cooler name than 'Front-falling Fireman's Carry Slam'.
So yeah, the RKO is probably one of the best, if not the best, finishers right now, because it fulfills all of these things. It's short, quick, hits the mat fast, and although it's just a cutter, it's got a name of power, which separates it from other cutters. No fancy stuff but can go into long setup if necessary.