Curious to see what people's opinions are. Fyi, I have not played Demon's Souls (though I would love to), and I hear DS has more punishing mechanics than DkS, so I'm wondering if that applies to its bosses.
Having only played DkS, my vote for sheer frustration goes to Bed of Chaos, but that's largely because of its poor design. Hardest "legit" boss fight for me would have to be Four Kings.
If there are some contenders in DS, I'd love to hear about them. Please try and avoid any spoilers regarding the story of DS, but as far as boss mechanics go I'm open to hearing them. --- Why can we kill Kalameet. He HAVE scales, so.. SHOULD he be immortal? Or, let's speculate here, why doesn't he IS anymore? - MikeoftheDesert
The Flamelurker in Demon's Souls gave me loads more trouble than any boss in Dark Souls ever did.
He has a similar melee moveset to Manus, (but with no magics) and moves at twice the speed... and he gets more and more aggressive and fast as you damage him. And his lightning fast punches make explosions the size and power of the ones that the Fire Sage Demon in Dark Souls makes.
Flame Lurker could demolish Ornstein and Smough in seconds flat.
Edit: Most every boss in the Souls series can give you a little breathing room to pop a healing item or cast a spell. Not Flamelurker. He is perpetually in your face, smashing right through your shield, and cooking you alive. --- The SAFE was slain in battle. A great flaming nautical pyre carries it off to VAULTHALLA. www.youtube.com/user/Terrafuror - Gimme a view, sweet cheeks ;)