For me, and of course this is just my personal opinion, based on my experience and preferences in bosses, there are a few contenders. I actually like a lot of the fights in the earlier part of this tier, maybe because we're used to them so they're more relaxed, but I like to think that it's because they have mechanics that are innovative without being gimmicky, and because the fights don't last forever. Some of the Terrace of Endless Spring fights seem to last about a week, and maybe it's because I'm healing, but that isn't a characteristic I particularly enjoy.
For me, as a shaman healer main, the best boss contenders are the following. Gara'jal the Spirit Binder, because the Spirit Realm health knockdowns, plus mana regen buffs, mean I can blow the top off the healing meter as mastery kicks in and mana becomes a non-issue. Pure /flexing, I know. And I must admit that it's pretty terrible on the raid finder.