##Actually, 26 bytes

    5Pτr`5Pτ;r)αZ♂M'AOi+♂cΣ`Mi

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Explanation:

    5Pτr`5Pτ;r)αZ♂M'AOi+♂cΣ`Mi
    5Pτr                        range(26)
        `5Pτ;r)αZ♂M'AOi+♂cΣ     for each element i:
         5Pτ;                     two copies of 26
             r                    range(26) (uses one copy)
              )α                  move to bottom, [i]*26
                Z♂M               zip the lists, pairwise maximum
                   'AOi+          add the ordinal of "A" (65) to each element
                        ♂c        character at each ordinal
                          Σ       concatenate
                            i  flatten list and let implicit printing do all the work