A Raiton enhanced pencil to the skull almost killed Kisame, regardless of Samehada's healing abilities. He is not going to be regenerating from decapitation, or from fatal attacks that will kill him instantly, like a Suidanha slicing through his brain.
With that said, Tobirama wins. None of Kisame's attacks have a hope of landing when Tobirama can comfortably sense them coming, and teleport out of their range. Neither are any of Kisame's attacks fast enough to overwhelm the reflexes of the guy who tagged Juubito and was canonically stated to be faster than an alive Madara. Water Dome is countered by Tobirama simply teleporting to a mark/marked kunai/marked shadow clone outside of the dome's range (which he would set due to full intel), and then using his natural speed to keep his distance, whereas he can counter 1000 Feeding Sharks in the same fashion, or with a large scale Suiton of his own. But I doubt Kisame will have time to perform such large scale attacks, since Tobirama with full intel would rather end things quickly.
Thus, I can see Tobirama using clones to pressure Kisame with Suidanhas and mark the battlefield, keeping him distracted before he throws his marked kunai in Kisame's general direction. If any kunai get close, he or a clone teleports in to decapitate him/stab through his skull with FTG Slash. In the event that Kisame is somehow able to keep his head intact, Tobirama or his clone slaps some explosive tags on Kisame and tries for a blitz again after they explode.
