The Collaborative International Dictionary of English v.0.48:

Upmost \Up"most`\, a. [Cf. Uppermost.]
   Highest; topmost; uppermost. --Spenser. Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

upmost
    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:

71 Moby Thesaurus words for "upmost":
   above, acmatic, acme, apex, apical, apogee, brow, cap, capital,
   chief, climax, cloud nine, consummate, crest, crown, culmen,
   culmination, edge, extreme limit, extremity, greater, head,
   headmost, heaven, heavens, height, high noon, higher, highest,
   highest pitch, highest point, limit, maximal, maximum, meridian,
   meridional, mountaintop, ne plus ultra, no place higher, noon,
   over, overmost, paramount, peak, pinnacle, pitch, point, pole,
   preeminent, ridge, seventh heaven, sky, spire, summit, summital,
   superior, supreme, tip, tip-top, top, topmost, ultimate, upper,
   upper extremity, uppermost, utmost, vertex, vertical, very top,
   zenith, zenithal