I feel this fact is super overlooked when speaking of Lich King's difficulty. He was a difficult fight, there is no arguing that. The gating however does take away a lot. You're given very little time to even practice any worthwhile strategy. Same shit with Algalon. I would personally never put him anywhere near the category of hardest bosses. You had bloody 2 hours each week to kill him, and his mechanics were pretty barebone. Tuning was tight, but then again so was Ra-den's for example.
I don't personally consider Lich King heroic 0% to be the hardest boss for the fact that it was probably the single most gated fight. Just so people are aware, the 20/25/30/35/40/45/50 pull attempts INCLUDED Putricide, Blood Queen and Sindragosa. You wipe on any of those 3 bosses, you have less for Lich King. There is a reason you've never ever seen the limited attempt thing repeated in the same fashion. It was super stupid, and basically meant the top guilds had a 2nd group of alts to practice parts of the fight, but again, different compositions and what not. Not as meaningful as actually practicing with a specific comp in mind.