Didn't do Naxx40 at level 60, but I did at least attempt the rest.
If you took a good group of 2016 raiders, put them on servers where each of those bosses were at their pomp, give them level appropriate gear & the tools available at the time, I reckon Kil'jaeden, Arthas, & Deathwing would go down pretty fast compared to Garrosh & Archimonde. They're mechanically-lite by 2016 standards, and many of the classes were brainless, even compared to today (I went through T4/T5/T6 content with a 0/21/40 Warlock alt that just put up CoD & spammed Shadowbolt).
Which was the hardest at the time for the playerbase? To me, a pre-nerf SWP clear carries the most weight.