Ryougi Shiki gets the win here. Her version of the Mystic Eyes of Death Perception allows her to "kill" concepts such as distance and destroy spells, as well as not causing her any strain from prolonged use like Nanaya and Tohno's versions. All three are about on par with one another in speed, but Ryougi's MEoDP is far more versatile with how she can dispatch the two split personalities.
She never killed distance in the novels.
It seems like one of my previous threads.
In my top five of most poweful characters, Ryougi is in first place, so I'm betting for her.
Tohno's eyes causes him pain, and also, he's almost all time trying to avoid conflict, so to a serial fucking killer like nanaya is easy prey
Ryougi doesn't want to kill, in fact the only one here with murderous intentions is probably nanaya
But, both, tohno and nanaya cannot kill any concept beyond physical or energy matters. So against a trained fighter with quick reflexes and perfect eyes able to kill all kind of concept like Ryougi, they are lost.
Nanaya is stated to be as quick as Ciel, so the faster here is Nanaya, but he'll probably attack Tohno first, because he will want to rid of his weaker side.
Ryougi won't kill anyone (maybe), but she may mutilate Nanaya's body, destroying the concept that joins his arms or legs by tracing the lines, however I don't belive she will attack this way Tohno.
Ryougi equipped with Kasenada Kuji, is with self suggestion and the third personality, in this way she is practically able to kill reality, also, one of her eyes can see the future while the other one can see the path to achieve that future, So she practically owns both quickly.
In resume, Tohno is fucked here, Ryougi will own Nanaya with her katana, and end of the story
Nanaya is never stated to be as fast as Ciel, that was half-vampire Shiki. Also where was it stated that Ryougi's eyes have additional properties that allow her to see the future (unless it was stated somewhere in Mirai Fukuin), and normal Ryougi never shows any precognition. Anyway, if Ryougi's third personality is allowed, then I guess she wins, as the author stated that she is the second strongest character in his major works.