This is an interesting question, because the way it's worded it appears the question is "what is [i]currently[/i] the hardest boss?" which naturally should be Skolas as it is the most current end-game content. However if we evaluate difficulty based on when it was considered "current end-game" content (Max light level was 30), then I would have to say Atheon, hands down. I sometimes end up in groups of all 34s, and it still ends up taking 30+ mins to kill Atheon and multiple wipes. Somehow people still don't know how to use the relic, or know that you have to shoot oracles when ported, or to throw a grenade when ported to Venus.. the list goes on. And again, I'm talking about the issues that I [b]still[/b] see. It was even worse when max attack power was only 300. That said, I think Skolas was pretty tough with arc burn but now that's gone. After the nerf I haven't needed to spend more than 1 or 2 wipes on him. I also feel some of the perceived difficulty comes from there not being a checkpoint before fighting him like there is with the other bosses. Finally, Crota is a joke, always has been. The only "difficulty" I ever have and ever will encounter with that mess of a boss was a result of broken mechanics (disappearing sword, Crota walking around where ever he wants to go, etc.) and lag (causing him to never go down, or stand up immediately after going down).