Antharas was indeed one of the hardest - because he wasn't really designed to be fought at that level. I think they tried with 500 people before any expansions came out, and failed miserably.
IIRC they defeated Antharas halfway through the second expansion, when people were sufficiently levelled up and had acquired more powerful gear. Either way, Lineage 2 bosses were massive clusterfucks of epic proportions due to no cap on how many people can attempt (they put one in later I think), in-battle resurrection spells with no casting limit, and the fact that they were made to take HOURS to beat. Not a sight you're gonna see for a long while in videogames.
Three of the bosses here are from WoW which is a bit... pandering. Speaking as someone who beat Algalon the Observer, he wasn't so bad and neither was Illidan (nevertheless Illidan caused a lot of drama due to his drops). LK just had an adjustable difficulty mode that went from easy to brick wall. It's kind of cheating if you ask me.
The only WoW bosses that should be mentioned is C'Thun which was undefeatable at launch due to bugs, and maybe Four Horsemen, the boss known as the "guild-killer" because people had so much problems that entire guilds would fall due to their best players leaving for better ones that could actually finish the fight. Otherwise, many pre-expansion WoW bosses took longer for people to defeat simply due to the fact that people weren't so used to the game back then (and design philosophies).