The Collaborative International Dictionary of English v.0.48:

Preeminent \Pre*["e]m"i*nent\, a. [L. praeminens, -entis, p. pr.
   praeminere to be prominent, to surpass: cf. F.
   pr['e]['e]minent. See Pre-, and Eminent.]
   Eminent above others; prominent among those who are eminent;
   superior in excellence; surpassing, or taking precedence of,
   others; rarely, surpassing others in evil, or in bad
   qualities; as, pre["e]minent in guilt.
   [1913 Webster]

         In goodness and in power pre["e]minent.  --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preeminent
    adj 1: greatest in importance or degree or significance or
           achievement; "our greatest statesmen"; "the country's
           leading poet"; "a preeminent archeologist" [syn:
           leading(a), preeminent]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "preeminent":
   absolute, acmatic, all-absorbing, apical, arch, ascendant,
   authoritarian, authoritative, authorized, autocratic, banner,
   capital, cardinal, central, champion, chief,
   clothed with authority, commanding, competent, consequential,
   considerable, consummate, controlling, crowning, dominant,
   duly constituted, eminent, empowered, ex officio, first, focal,
   foremost, governing, great, head, headmost, hegemonic,
   hegemonistic, highest, imperative, important, incomparable,
   influential, leading, magisterial, main, major, master, maximal,
   maximum, meridian, meridional, mighty, momentous, monocratic,
   number one, official, outstanding, overmost, overriding,
   overruling, paramount, potent, powerful, predominant, premier,
   preponderant, prestigious, prevailing, primal, primary, prime,
   principal, prominent, puissant, ranking, ruling, senior, sovereign,
   star, stellar, substantial, summital, supereminent, superior,
   supreme, surpassing, tip-top, top, topflight, topmost,
   totalitarian, towering, transcendent, ultimate, unmatchable,
   unsurpassable, upmost, uppermost, vertical, weighty, zenithal