Tricky one. From a lore perspective, i'd say Manus. For one thing, he's the furtive pygmy, meaning he's in possession of the dark soul. Admittedly, he may have lost some of it (got turned into humanity) but he must still have a fair portion of it, not to mention that his soul grew in power over time, unlike the other lords. Given that he's powerful enough to take down Artorias and all of Oolacile (and would've succeeded if not for the player's intervention), I'd say he's powerful enough to kill Kalameet without too much trouble.
Gameplay-wise i'd say Manus again. Kalameet has some serious magic defence, but so does Manus (Kalameet's breath does magic damage, not fire). In addition, Kalameet is too big to evade Manus' dark magic, which are some of the most powerful boss attacks in the game. In particular, his attack where he sprays dark beads out in a cone in front of him does tons of damage to anyone unable to get out of the way.
Overall; Manus wins this one.