To the Draenei, I give the title of most religious. The Draenei have a set of beliefs that's been relatively unchanged for over twenty-five thousand years. Their civilization is a theocracy led by a prophet. The civilization has a clearly defined, even if lofty, goal that is indistinguishable from their religious beliefs. Following that line of thinking they also have a Theo-military run by those with divine power.
To the Trolls, I give the title of most spiritual. Trolls have a very diverse and incredibly adaptable spiritual system. I proclaim them the most spiritual, simply because Trolls don't have make distinction of anything being non-spiritual. They (rather correctly) believe that things such Technology and Arcane magic, which is usually shoved in the category of being purely academic, to also be spiritual. As technology is another way binding the elements, and arcane magic also affects spirit realm. They have deities, omens, and rituals for nearly everything, turning a usually non-secular activity such as making a weapon into a spiritual endeavor by performing voodoo during the forging process or having their hunters take on the aspects of a certain Loa.
They're system makes them quick to adopt any new divine entities as their own(such as animals spirits in outland and the Ancients in Hyjal) and also non-averse to incorporating new spiritual practices (like The Light or Druidism which was always quasi-present).
When contrasting Dreanei and Trolls, I think the distinction is pretty easy to see. Trolls don't seem to have a hard-lined doctrine, probably passing down most lessons through orally through fables, making them more flexible as even Dreanei were a bit averse at first to the practice of shamanism which had apparently evaded them even when they came into contact with it. While Dreanei have a goal to work to, Troll religion doesn't have a purpose, even becoming a Loa isn't a goal anyone is seen striving towards.
So now I'm left to explain why I don't think Tauren or Night Elves deserve those titles over Trolls and Dreanei. Taurens are rather easy. Yes, they have the whole wise, peaceful, native thing going, but being wise and peaceful doesn't necessarily make them more spiritual, at least in comparison to trolls. Again, even Dreanei and Tauren don't consider everything a spiritual endeavor. Troll Voodoo very much blurs the line, affecting deity, natural, and arcane magic, infecting things such as alchemy and being ingrained to the point where mages and shaman perform and teach together.
Night Elves are harder. Firstly, they're a double theocracy. Both sides of society technically bent in eternal penance to the protection of nature. Dreanei only win out because they are actually striving to something, being active not reactive. Its also something everybody in some way is concerned with, even mages. Night Elves have kind just spent a lot of time existing, especially the citizenry.
Trolls win out because they're devotional while Night Elves are more often inwardly contemplative. A Night Elf smith can seek out ingredients, study the tigers, and think about the natural song metal makes when he's hammer a glaive, but you can contest whether that spiritual or not. The Troll's sword on the other hand, literally has the spirit of a tiger in it.