What is the most powerful mouth closer in the human? The masseter or
the temporalis? I've asked various prominant dentists in academia and
there doesn't seem to be a definite answer. I'd like answers from
research articles or established institutions such as the American
Dental Association.

I know I'm not providing references, but (in my opinion) it has to be
the masseter for sure. Although I don't know which muscle is stronger
since they seem to be approximately the same size (I believe it's the
masseter also), the leverage that the masseter has is far greater than
the leverage of the temporalis. The masseter pulls perpendicular
between the mandible and the maxilla, whereas the temporalis pulls
more parallel to the line of motion than perpendicular.

I'm leaning towards masseter as well but I was wondering if I could
get a solid answer from a study or an established source that was
pretty confident. I got a response of "masseter... I guess" from
people who generally make it their business to know such arcane facts.
Thanks for the insight though

Does this help?

"A) Masseters are the strongest muscles of mastication in herbivores. 
    "Function and Morphology of the Temporomandibular Joint According
to Eating Habits" from Anatomical Atlas of the Temporomandibular
Joint, Y. Ide, K. Nadazawa, Quintessence 1991
 B) In humans, the masseter is the second most efficient masticatory
muscle, with a ratio of neurons to muscle fibers of 1:600.
    Carlsoo S: Motor units and action potentials in masticatory
muscles. Act Morphol Neerl-Scand. 2:13-19; 1958
 C) The temporalis' % strength of maximal bite force is greatest when
the mandible is open ~20 mm.
    Effect of jaw opening on the direction and magnitude of human
incisal bite forces. Paphangkorakit J; Osborn JW, J Dent Res, 1997
Jan, 76:1,561-7
    See also: 
      The effect of pressure on a maximum incisal bite force in man. 
      Effects on human maximum bite force of biting on a softer or harder object. 
 D) In carnivores (those animals with prominent canine teeth) and in
humans, the temporalis is the strongest and most efficient muscle of
mastication. (A, above) The ration of neuron to muscle fiber is
1:900."

http://www.nti-tss.com/refrnce.htm