I was thinking Ornstein and/or Smough, but Ornstein's armor is made of Gold, and I'm assuming Smough's is as well. Since gold is softer than Iron, then even their giant forms, which aren't as big as the golem, O&S wouldn't make it.
I was considering Artorias, since his armor seems pretty sturdy, but even if it was a somewhat tougher material, the sheer size/ thickness of the Iron Golem's armor would probably make up for it.
The Gargoyles seem pretty brittle as their are, so certainly not them.
Most other bosses seem to be pretty flesh-y, so that leaves them out as well.
Even the strongest Dragon/ drake scales should be no match for giant thick plates of iron, specially since you can easily cut off their tails.
So yeah, it seems Iron Golem is the hardest boss.
Now the real question is who's the softest boss, Priscilla or Sif?
This has to be an April fools joke, right? ^^
Haha, you know, I should have waited a day and made it that. :P