Shunsui killed the strongest Espada with just his Shikai. All other captains had to use their Bankai plus more to defeat the lower Espada.
Shunshui is more powerful in literally every way except for physical strength when it comes to comparing him with Kenpachi. His Zanpaktou was not even letting him fight properly because "she wasn't in the mood" and when she did finally want to play, Stark stood absolutely no chance.
Shunshui does not need to tell Kenpachi the rules of his Shikai games. Irooni ends him as Kenpachi has no idea how to fight against it and one attack is all Shushui needs to kill him.
By feats: Shunsui >> Kenpachi
By Hype/Portrayal: Shunsui >>>> Kenpachi
For a fact, Zaraki is SWP. That alone speak for itself.
Kyoraku's power is already mature, and for him to make in SWP list is very unlikely, considering that there exist strongest mature shinigamis (RG, Urahara, Isshin, probably Byakuya) and some very talented young Shinigamis like Hitsugaya.
That's only counting Shinigamis, so Kyoraky will very likely not be SWP, which prove to us portrayal - Zaraki >>>> Kyoraku.
Feats? When Kyoraku in shikai cut meteor than we can compare their feats for know Zaraki stomps.