Supreme, superlative, transcendent, surpassing, preeminent, peerless, incomparable can all mean highest in a scale of values.
All of these words may be interchangeable when used rhetorically or bombastically with the resulting loss in definiteness, but all are capable of discriminative use in which they carry distinctive implications and connotations. Supreme is applicable to what is not only the highest in rank, power, or quality but has no equals in that status, all others of the same class or kind being inferior in varying degrees.
Superlative is applicable to whatever, by comparison with all other things of the same kind or with all other manifestations of the same quality, admits of no superior especially in commendatory qualities, for superlative may admit equals but it excludes superiors.
Transcendent and surpassing are applicable to whatever goes beyond everything else of its kind or in its quality; both can connote an exceeding even of the superlative, but transcendent suggests realization of the ideal, and surpassing suggests almost inconceivable attainment.
Preeminent is applicable to what goes beyond all others in achieving distinction or eminence; it implies both superlativeness and uniqueness within the limits indicated, but it seldom carries a suggestion of direct supremacy or transcendency.
Peerless and incomparable both imply the absence of equals but, commonly, peerless connotes the absence of superiors and incomparable connotes the impossibility of being equaled; while both normally refer to commendatory qualities, incomparable is also freely referable to qualities that merit condemnation.