So far for level 90s I have: Monk (main), Mage, Lock, Shaman. I wont attempt to comment on other classes.
Of these classes, I've tried every dps spec. I will say:
The hardest imo was Demonology Warlock, with WW monk being a close second. What I find makes WW monk hard is optimal use of Fists of Fury, and the fact that at higher gear levels, energy capping is hard to avoid.
The easiest is by far Elemental Shaman, no contest. Especially when taking the Primal Elemental talent at 90. You can make it slightly more complex by adding Elemental Blast, but why bother. Close to being as easy as Elemental Shaman for me was Destro lock and every mage spec. The only even slightly hard thing about the mage specs is working with your level 90 talent, but I imagine that gets easy with practice too.
As BM hunter myslef - NOT a BM hunter. Right now it's a flacking whack-a-CD thing you can't even call a rotation. On the other hand, if you just go afk, you can get 15-20k dps unduffed from just autoattack and procs.
The easiest one I have encountered so far is rogue. I have played combat, but from what I've heard others are not much different.
Then there's ret pala, on which I have to admitt I have fallen asleep a few times during LFRs.
I think it all depends on what you have feel for, straight jumping in id say 2H frost DK
balance druids hands down...
I pretty much agree with Mut rogue, very simple, just maximizing envenom uptime.
There is more management with combat now than there used to be with anticipation and holding CP's for the next lvl insight(and of course using killing spree without killing yourself while still trying to fit in a small window and getting extra dmg)
Arathian Knights US 3RD
Cast big nuke on CD / proc.
Manage your resource (shadow orbs, embers/soul shards, fulmination stacks, eclipse energy, etc..).
Macro big CDs together and use with hero / trinket procs / on CD.
All classes have some intricacies (generally around resource and CD usage), but the template is the same. I don't see a marked difference in difficulty amoungst the range classes (granted, don't have a max level hunter).