Kisame > Sasori > Kakuzu >= Deidara.
KISAME IS THE BEST HERE
Kisame has easily proven to be the strongest of the 4. The other 3 use ninjutsu based attacks, great Shark bomb can absorb Kakuzu's heart abilities, Sasori's iron sand/fire/water style, and Deidara's clay (which uses chakra to explode!!!). I'd really hate to go in depth, but GSB ends the discussion here so I'd rather leave it here unless someone has a good counter for GSB they can bring up.
SASORI Vs KAKUZU
Sasori > Kakuzu. This one could potentially go to Kakuzu, but Sasori has much much better winning chances. Sasori's iron sand, fire/water style have enough feats to take the 5 hearts on. While Kakuzu does have the advantage with more elements here. Iron sand will only be stopped by the suiton heart. Sasori's Suiton feat is much greater than Kakuzu's suiton feat. Sasori took out a rock, Kakuzu's suiton never really did anything. I will not be including poison in this since Kakuzu's body was never really clarified by Kishimoto. Domu is easily stopping a bunch of poison needles though. Sasori also has 100 puppets of fodder on top of the jutsus I listed earlier that could stalemate Kakuzu. Kakuzu could beat Sasori, but Sasori has more proof of being able to win.
Kakuzu/Sasori/Kisame > Deidara
Deidara's suicide is irrelevant here. If Deidara had to suicide to beat the others, then I would have put = instead of >. Suicide is not a win, it's a draw.
While Kisame's attacks have not shown to be able to hit Deidara, that's okay. Deidara's attacks can't hit Kisame. C4 will get absorbed by GSB. Sasuke stated he saw the nano clay explosives because the Sharingan can see chakra. Guess what GSB absorbs? Chakra. Kisame easily has more stamina as well. Deidara is being out lasted here if it stalemates. If anything, Deidara's character has shown the fight will make him frustrated and tired. Thus he decides to result in suicide once Kisame gets the upper hand. I may be jumping to conclusions quickly but, If the clay detonated simply because he infused the clay with chakra. The same would go with Deidara's suicide attack. He uses chakra to explode. Therefore GSB can eat it up.
Deidara admitted inferiority to Sasori. Now Deidara may have said this, but it's not as if they ever had a deathmatch. Sasori as a puppet himself can fly, along with his 100 puppets. C2 might cause trouble because Sasori tends to lack agility. Iron sand can tank Deidara's attacks if Gaara was able to. Sasori also has the fire and water style that can go long range and wipe out Deidara's clay bird. C4 is doing nothing because Sasori doesn't breathe. Deidara is not going to suicide here because that is a last resort technique. Sasori is the type of fighter that can take out Deidara before he gets a chance. This is because of poison. One scratch of poison numbs Deidara's body.
Kakuzu could potentially lose here. He is prone to Suicide from Deidara, it's a matter of Kakuzu taking Deidara out ahead of time.
Kakuzu's taijutsu feats have proven he is agile enough to dodge standard explosives. His 5 hearts can fly so Deidara flying is out of the question here. C2 is stopped by the lightning heart. The lightning heart + fire/wind heart combined can overwhelm Deidara easily. Deidara cannot use C4 here, because the 5 hearts can fly and will prevent Deidara from being able to get remotely away from the C4 blast range. It Deidara is killed overwhelmed and killed by the hearts, Kakuzu obviously wins. If Kakuzu tries to drag it out and rip Deidara's heart out, Deidara would kill Kakuzu resulting in a draw. Since Kakuzu has much more potential, Kakuzu will most likely win. This boils down to a draw. Deidara is not beating Kakuzu and surviving.