For the most part it's been pretty consistent with item level.
I've completed:
Mage (all)
Rogue (all)
Priest (all)
Druid (all)
Shaman (all)
WW/BrM monk
Ret pala
I've completed 19 out of the 36 challenges and by a massive, massive margin the hardest was the first I did it on; frost mage
Done at 894 Ilvl with a lack of traits in my weapon (not having instant blizzard made it way harder) and lack-luster legendaries, I just did not have the damage to deal with the mechanics - Didn't realize you could move your pet onto the void zones at the time either. Took me over 100 tries, nothing past that has took me more than 20 and have not attempted a challenge with higher than 920 ilvl (cbf gearing alts, just want the mog in case I main them in the future).
Out of the list above the hardest were probably the healing ones, it just drains you mentally having to start it over when it's that long. Easiest has definitely been any Tugar fight and the ret/assassin/enhance Sigryn challenges. Ironically, I struggled a lot more on my mage (which was my main, and had more gear) with Sigryn than I did on any of my undergeared melee alts. I feel like the tank challenge is nowhere as bad as people make it out to be, I've not done the dreaded prot war/pala ones yet but having done Guardian and BrM fairly painlessly with 1 random legendary each and < 910 ilvl, I feel like i'll not struggle too much.
As a side note for those wanting to do the affli challenge, you don't need to have the ring or cheese it. I recently watched my friend do it without either simply by making good use of self healing and burning rush; You don't need to cheese it, pretty sure there's a few videos online. Just thought i'd throw that out there as I've seen it mentioned here a few times that it's impossible otherwise.
Can't wait to get the rest of the challenges done, be cool if there was a feat of strength for it, just need to get a little bit more gear on my alts (half of them sat at like 870, gearing up a new alt kills me).