There's too many variables to count difficulty tbh, from a pure dps perspective probably hc LK at least early on, because his enrage timer was actually hard to beat despite it being 15 minutes long, mechanically he was probably one of the easiest end bosses though. I personally don't think HC LK was that hard and given time I think plenty of guilds would've killed him had they not added the dumb limited attempts as well the nerf over time.
The original KT was pretty brutal, I never killed him before BC launched but had many sub 20% tries. They had never really launched anything as difficult as this at the time in terms of mechanics, strict dps reqs etc (don't you dare even say 4h, getting 8 geared tanks is not mechanically difficult). Wouldn't rank it as the hardest but it definitely beats some of them.
Madness 25hc was a joke, it was probably one of the easiest bosses in the whole instance.
Never done KJ before he was nerfed, opinion means moot.
Garrosh 25hc was scripted and easy as fuck, came down to a few people doing their jobs correctly and sadly for the Guild I was in at the time our Brewmaster was mentally impaired and found it difficult to pull an Iron Star in to the boss. Only thing that Garrosh had going for him was his monstrous Tank damage, but even then none of your wipes generally came down to Tank deaths if they knew what they were doing.
Archimonde before the ilvl buffs and capped rings is probably what I'd consider the hardest, all of his mechanics were extremely punishing and you couldn't just cheese them early on, strict dps reqs for the Doomfires & Infernals, limited conduit spacing near the end of P3 and some pretty brutal HPS reqs for almost all of his Raid damage mechanics. I still envy Method for managing to kill that without any Leg rings.