My vote is for Flamelurker. Last time I played through, I avoided him because I remember his aggressiveness. Especially when you hit ng+. I don't ever have a hard time with Gwyn. It seems like if you keep hitting him, he just stands there. And yes, there is that one glaring weakness.
Well I am doing my first Demon's souls playthrough at the moment. My build is pure melee. I beat him in my 4th try,and I have to say that the worst part of the battle is when his life is around 1/3 or 1/2. At the end he just spammed his explosive attacks,which are hilariously easy to dodge.He was intimidating surely,but if you are calm you have nothing to fear.
"I think they are about the same level of aggressiveness, although I'd compare Gwyn to Allant"
If you make THAT comparison, then Gwyn looks like a friggin third grader next to the False King, he's still a beast to this day. Though I don't find the Lurker or Gwyn particularly troubling today, the FL still echoes the old intimidation I felt the first time playing the game through. Also, he's like the fourth boss you encounter in demon's souls? Gwyn was the last, and by that point I was tough enough not to sweat him.
I recall actually being able to heal against Flamelurker without using something between us to hold him off. With Gwyn, I either have to get some pillar of rock between us and pray, or parry him to give me just enough time to heal.
I am snazzier, hot, hot rant. Warily slight as. Croak rush, OK? Weirder, almighty make out. ::)
My vote is for flamelurker. Beat every boss in DeS first try, including false king, although that was probably because i had beat dark souls 98467 times before this. But man, freakin flamelurker just don't give no breaks