I'm going to have to agree and say Kankuro although it would be a tough one. If I'm correct than his Black Ant puppet has poisonous daggers/kunai that it can either shoot out or stab with. He can also rig his puppets with various traps and manipulate them furthermore, including the head of some puppets. Traps like poison gas bombs can easily be fatal if the enemy is caught off guard (full knowledge would help Sai here).
Sai is shown to have the skillful stereotype and he doesn't waste moves. That's why I doubt the Black Ant puppet is going to be enough to take on Sai. His Crow puppet could be used as a trap though, and skilled enemies have fallen for it before (see Kankuro vs. Sakon/Ukon and his fight against the ambush squad). Sai doesn't have the strength to bust out of there, so he's pretty screwed if that does happen to him. The Salamander could also provide defense for some attacks even though it's pretty feat-less in the manga.
So Sai's best chance is to hop on an ink bird and try to strategically break Kankuro down. Honestly I don't think Sai's fully capable of escaping danger just by being on his bird, since Kankuro's Sasori puppet has wings capable of flight and long-range attacks like its flamethrower and extendable metal wire (whatever you'd like to call it). It's also been shown to have chakra shields, which should help against exploding ink birds from above eg. against Zabuza+Haku.
Sai could possibly go rage mode in this fight and blitz Kankuro with the ink giants in this fight but not only would he have to be close enough to do it, but it might not work with Kankuro's hidden puppets and defenses set up.
