The Collaborative International Dictionary of English v.0.48:

Inimitable \In*im"i*ta*ble\, a. [L. inimitabilis: cf. F.
   inimitable. See In- not, and Imitable.]
   Not capable of being imitated, copied, or counterfeited;
   beyond imitation; surpassingly excellent; matchless;
   unrivaled; exceptional; unique; as, an inimitable style;
   inimitable eloquence. "Inimitable force." --Dryden.
   [1913 Webster]

         Performing such inimitable feats.        --Cowper.
   -- In*im"i*ta*ble*ness, n. -- In*im"i*ta*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inimitable
    adj 1: defying imitation; matchless; "an inimitable style"

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "inimitable":
   beyond compare, beyond comparison, easily first, facile princeps,
   immortal, incomparable, invincible, matchless, nulli secundus,
   peerless, second to none, sui generis, unapproachable,
   unapproached, unbeatable, unequaled, unexampled, unexcelled,
   unique, unmatchable, unmatched, unparagoned, unparalleled,
   unpeered, unrivaled, unsurpassable, unsurpassed, without equal,
   without parallel