Depends on where the fight took place, anywhere besides crystal cave, Seath would be immortal, Sif would be the strongest due to it's mobility. AS SL1 runs proved, mobility is everything, if player can roll through all boss attacks, Sif can definitely do as well as one of it's leap is probably 10 player rolls.
I'd say 4 Kings, your talking about one boss that has 1500 or so HP and, if the other bosses don't kill one fast enough, will have an ally summoned, then another, and another.
However in the sake of being creative Seath the Scaleless might actually be a extremely viable contender. Before you start laughing think about it, it's the only boss that can, no matter how much HP a character might have, OHKO if curse builds up enough. Now consider the fact that since absolutely NO weapon, item, spell, or ability the character can utilize has a curse effect, EVERY BOSS IN THE ENTIRE GAME COULD POTENTIALLY BE CODED WITH A 0 CURSE RESISTANCE. Afterall, why give bosses a resistance to an effect the player can never use against said bosses? In theory this means that Seath would only need to spread crystals under the bosses once and instantly kill every single boss in the entire game, save for Moonlight butterfly and Bed of Chaos
Manus, just imagine his dark orb attack where all of them end up hitting the target due to the other bosses sizes. Its likely the highest potential damage of any one bosses attacks, especially against other bosses.
Heh artorias based off his mechanics would be one of the few that may stand a chance, which is funny since manus beat him already.