Kisame Hoshigaki
Sasori has the third Kazekage and his sand, Hiruko, and his Red Secret Jutsu which gives him control of a 100 puppets. He also has scorpion, which used to be his real body until he turned himself into a chakra core which contained his chakra and conciousness (Spelling sorry), allowing him to move from puppet to puppet, essencially Sasori is that chakra core and nothing else. Now that makes Sasori extremly powerful but when it comes down to it both shinobi know of the others fighting style; Kisame being a power house using water techniques, Samehada, regeneration, and his ability to merge with his blade. Sasori with his puppets and knowing of his poision (which is used by all puppeteers) and the hidden compartments. I say Kisame wins this battle for theses reasons:
Samehada has the ability to each chakra and feed it to Kisame to replenish or add to his already large amounts of chakra. Sasori's puppets are controled through chakra strings meaning, even though Sasori could send more chakra into his puppets Samahada keeps eating it and giving it to Kisame. Sasori give his chakra to Kisame ultimatly making him stronger. Now Sasori's Kazekage puppet controls the magnetic sand, which is also controled by chakra again feeding Kisame. Next of all for Sasori's speed and unpredictable attacks, Kisame is fast as well and knows of Sasori's core and how it can change bodies. What's more Kisame also has long ranged water ninjutsu to submerge Sasori's puppets if he decides to fight long ranged. Sasori's water shooter and flame thrower are strong but Samehada's isn't weak enough to get cut through like the rocks Chiyo and Sakura hid behind. As for the flame thrower, if Kisame used his water jutsu, especially his large dome the flames would eventually be overwhelmed. Kisame's speed under water is much greater then his on land and I'm not sure if Sasori was caught in water I'm not sure how he'd manage against Kisame. Both fighters fight at long and close range but I just see Kisame coming out above Sasori on this one.