WLS hasn't been a "top-tier" set up for a while. It was very strong at the start of the season, and now I would almost rate it second tier because there are so many mage/warrior, rogue/mage, turbocleaves, kitty cleaves, and KFCs that literally just demolish it. Not to mention the plethora of other random melee cleaves that can win simply by training your Shaman from start finish.
I've played with both Spriests and Boomkins this season on some of my healers, and although Spriests are shit - Moonkins are way worse. And I mean waaaay worse.
The main difference between the two is damage. They both die equally as fast, and they both get face rolled by warriors and other melee, but if you keep a Spriest up there's a chance your team might land a kill. Shadow Priests can actually "burst". You can set things up with them, and their damage is nothing to scoff at.
Boomkin mechanics don't allow this. They are 100% reliant on procs, and not in the good way. Their damage is hardly ever there when you actually need it. Setting up a kill? Too bad! I didn't get any procs! Maybe next time! This is why Boomkin/DK is one of their only "good" comps. The Boomkin just acts like a second healer, and random proc damage is fine when your DK can solo things anyway, so long as they have support to do so. Throw a few clones here and there, offheal like crazy, and coordinate solar beam with soul reaper. The downside of the comp though, is you're playing with a Boomkin and DK (lol).
Also, Warlocks are good - they just suck against our mage/warrior/rogue overlords. Sucks they also get shit on by hunters though, but the majority of classes do as well - they just have the same problem you do with said god classes.
Also, what about those pesky Kittycleaves that show tons of restraint with their interrupts and literally let you juke yourself to death? Sometimes the damage is so high that you can only afford to juke maybe once or twice before casting is no longer optional.
And I can't deal with mage/rogue at all if they are training me. I eventually run out of CDs and die, even with peels and gateway. If they are on a partner it can be very hard not to trinket early, and once I do I'm screwed next dance. Seems like our hardest counter... Would rather fight warrior/mage all day.
Boomkin/DK/RSham or MW can work. Kinda.. Seems decent against caster cleaves and caster/melee comps but any melee cleave or warrior/hunter cleave is going to push your shit in.
Shadowcleave with a MW or Paladin is decent. Would probably work with a Druid too, since that class works with everything. Doubt it would work well with a Shaman because your peels would be insufficient against any "train the blue" comp.
PHD would be good.
Double DK is great if you're only quing into RShamans (lololol).
DK/Mage would work, because... Mage.
DK/Rogue works decently, especially again if you're quing into a lot of RShamans.