WordNet (r) 3.0 (2006):

unbeatable
    adj 1: hard to defeat; "an unbeatable ball team"
    2: incapable of being overcome or subdued; "an invincible army";
       "her invincible spirit" [syn: invincible, unbeatable,
       unvanquishable]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "unbeatable":
   beyond compare, beyond comparison, easily first, excellent,
   facile princeps, immortal, impenetrable, impregnable, incomparable,
   incontestable, indomitable, inexpugnable, inimitable, insuperable,
   invincible, inviolable, invulnerable, irresistible, matchless,
   nulli secundus, overpowering, overwhelming, peerless, resistless,
   second to none, sui generis, superlative, supreme, unapproachable,
   unapproached, unassailable, unconquerable, undefeatable, unequaled,
   unexampled, unexcelled, unique, unmatchable, unmatched,
   unparagoned, unparalleled, unpeered, unrivaled, unsubduable,
   unsurmountable, unsurpassable, unsurpassed, unyielding,
   without equal, without parallel