The Collaborative International Dictionary of English v.0.48:

Unforgettable \Un`for*get"ta*ble\, a.
   Not forgettable; enduring in memory.
   [1913 Webster]

         Pungent and unforgettable truths.        --Emerson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unforgettable
    adj 1: impossible to forget [ant: forgettable]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "unforgettable":
   celebrated, conspicuous, distinguished, eminent, esteemed,
   estimable, exceptional, extraordinary, haunting, indelible, marked,
   memorable, nagging, noble, notable, noteworthy, obsessive, of mark,
   outstanding, persistent, prestigious, prominent, rare, recurrent,
   remarkable, rememberable, reputable, salient, signal, special,
   striking, telling