Demon's is the best
There are really only three really hard bosses in the game - Flamelurker, Maneater and the False King, I think.* Of course all the others vary in difficulty according to your build, but I can't think of anyone really hard apart from those three.
Similarly, the 'horrible' levels such as 5.2 and 3.1 really go up in your estimation as pure genius the more you play them.
Q: in DSII, how did you deal with the Iron King and the big frog with no magic/ranged stuff going on? I'd imagine they would be quite difficult as melee-only.
The Embedded eh? Horrible.
* That said, I am currently stuck on Penetrator.