Tsunade - High chakra reserves used solely to augment Taijutsu and heal wounds she may aquire in battle. Samehada hits her once if she goes all out (with Yin Seal: Release), and it's done for because her body automatically sends chakra to the wound where Samehada eats it up. Both of them are proficient in close-combat, but when Kisame get's enough chakra, Tsunade becomes an old granny who can barely move anymore. Considering she has no ranged attacks except ground-destruction techniques (which Kisame can counter with water), she's screwed.
A - Kisame can take him the same way he took out Bee. If A is determined to kill Kisame, without regard to his own life, A would electify the water dome when Kisame goes Shark-Mode, killing the both of them (assuming Kisame can't absorb that lightning before he gets electicuted). If A wants to take care of his own life before killing Kisame, he's screwed.
Gaara - Gaara's sand isn't nessicarily weak to water, however, it will be slowed down by great amounts of water, just like human movement is. It's only truly weak to 2nd Mizukage's Oily Water. However, if Kisame gets serious and uses Shark-Mode against Gaara, he'll be in the Water Dome. Gaara's sand will be slowed, and Kisame will be able to move incredably fast, hit Gaara once, steal his chakra, and Gaara will drown. Gaara is an ex-jinchuriki, and Kisame was an expert in taking Jinchuriki alive, which is harder than taking them dead.
Mei - Steam style essentially renders a great deal of Suiton useless. But then again, Samehada absorbs chakra from anything by touching it. This is a difficult battle, and unlike the other's, I don't think Kisame has the option to go Shark-Mode (because Mei can evaporate all of the water, or even use it to her advantage). Using Lava release when Kisame sends water towards her creates stone walls for defense from further water techniques, and overall she's a tough opponent for Kisame. This is a hard one... However, as I see it, this fight will come down to who can hold out the longest with chakra reserves, which is something Kisame can do better than Mei (unless there's proof otherwise).
Oonoki - Dust Release is the hardest part of this fight, so I'll leave it for last. Kisame can counter the flight by creating a lakes-worth of water and then creating large collums to stand on, raising himself higher and higher into the sky; if Oonoki spends all of his time running, it'll eventually end bad for him (though he'd be able to solely run for a while). After that, all we have is Oonoki's Jinton versus Kisame's skill. He has three very useful chakra-absorbing, but only one which could be of use here, IMO: Great Shark Bomb (in case you're wondering, Samehada, the general Chakra Absorbtion technique which he can use without Samehada, and GSB). Sending this massive shark outwards, which Kisame was confident enough to be able to absorb the chakra from Gai's "most powerful technique", should be able to eat up the chakra from Jinton; even if it gets destroyed while eating up the chakra, Kisame could use that oppertunity to attack as his hands and chakra reserves are free, while Oonoki's are busy until the technique is made useless. Ultimately, if Kisame has a chance here, it's high difficulty. But he also has great enough intellegence to be able to come up with a plan to get around Jinton (assuming the Shark Bomb isn't enough).
Really, Oonoki and Mei are his two toughest opponents.
