The Collaborative International Dictionary of English v.0.48:

Topmost \Top"most`\, a.
   Highest; uppermost; as, the topmost cliff; the topmost branch
   of a tree.
   [1913 Webster]

         The nightngale may claim the topmost bough. --Cowper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

topmost
    adj 1: at or nearest to the top; "the uppermost book in the
           pile"; "on the topmost step" [syn: topmost,
           uppermost, upmost]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "topmost":
   acmatic, all-absorbing, apical, best, capital, cardinal, chief,
   consummate, controlling, dominant, first-class, first-rate,
   foremost, greatest, head, headmost, highest, leading, main, master,
   maximal, maximum, meridian, meridional, most, outside, overmost,
   overriding, overruling, paramount, predominant, preeminent,
   premier, primary, prime, principal, ranking, summital, superlative,
   supreme, tip-top, top, top-notch, ultimate, upmost, uppermost,
   utmost, vertical, zenithal