The Collaborative International Dictionary of English v.0.48:

Preponderant \Pre*pon"der*ant\, a. [L. praeponderans, -antis:
   cf. F. pr['e]pond['e]rant. See Preponderate.]
   Preponderating; outweighing; overbalancing; -- used literally
   and figuratively; as, a preponderant weight; of preponderant
   importance. -- Pre*pon"der*ant*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preponderant
    adj 1: having superior power and influence; "the predominant
           mood among policy-makers is optimism" [syn: overriding,
           paramount, predominant, predominate,
           preponderant, preponderating]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "preponderant":
   arch, ascendant, at the head, banner, boss, capital, cardinal,
   central, champion, chief, commanding, controlling, crowning,
   dominant, first, focal, foremost, general, governing, great, head,
   headmost, hegemonic, hegemonistic, in ascendancy, in charge,
   in chief, in the ascendant, leading, magisterial, main, master,
   on the throne, overbearing, overruling, paramount, predominant,
   predominate, preeminent, prepollent, preponderate, prepotent,
   prevailing, prevalent, primal, primary, prime, principal, ranking,
   regnant, regulating, regulative, regulatory, reigning, ruling,
   sovereign, star, stellar, supereminent, supreme, swaying,
   topflight