The Collaborative International Dictionary of English v.0.48:

Sui generis \Su"i gen"e*ris\ [L.]
   Of his or its own kind.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sui generis
    adj 1: constituting a class of its own; unique; "a history book
           sui generis"; "sui generis works like Mary Chestnut's
           Civil War diary"

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "sui generis":
   another, beyond compare, beyond comparison, breakaway,
   easily first, else, facile princeps, immortal, incomparable,
   inimitable, invincible, matchless, not that sort, not the same,
   not the type, nulli secundus, of a sort, of another sort, of sorts,
   offbeat, other, other than, otherwise, peculiar, peerless, rare,
   recherche, second to none, special, unapproachable, unapproached,
   unbeatable, uncommon, uncustomary, undreamed-of, unequaled,
   unexampled, unexcelled, unexpected, unfamiliar, unheard-of, unique,
   unmatchable, unmatched, unordinary, unparagoned, unparalleled,
   unpeered, unrivaled, unsurpassable, unsurpassed, unthought-of,
   unusual, unwonted, without equal, without parallel