I'm conflicted, mainly because I like Tohno more than Ryougi simply as a matter of taste, and I can see this going either way, really.
Tohno has superior AP and durability, though in a battle between MEoDP users those hardly matter. The biggest deciding factor (speed) is equalized, making it an even closer match-up. Since this is base Ryougi vs Peak!Tohno (Satsujinki), I'm inclined to go for Satsujinki simply because of his feats of incapacitating a member of the Burial Agency (someone comparable to Ciel) and managing to kill a Dead Apostle Ancestor before he could react to his presence.
Tohno as Satsujinki definitely has far superior stealth, managing to get through the security of aforementioned DAA with seemingly little effort, and with the 1000-metre starting distance, he could logically set up an ambush and avoid direct combat altogether.
Of course, if we're not counting Satsujinki and instead just Nanaya... once again, it can go either way. Nanaya is just Shiki's mind focusing entirely on the act of killing his opponent, and tapping into the skills he possessed prior to his amnesia. He isn't quite unconscious, just that his killer instinct overpowers every other thought process. So the real question is, would Nanaya go for an assassination, or charge into direct combat? If he goes for the assassination, then Tohno wins with little difficulty since Ryougi had no precog without her sword. If he goes for direct combat, Ryougi wins with major difficulty due to being able to outlast Tohno since her Eyes don't tax her.
This is just my opinion on the subject, but I think that Nanaya would go for the assassination, so count my vote on Tohno Shiki as the victor.