##Actually, 19 bytes

    5Pτ;r;∙`MúEû`M╪k♂Σi

[Try it online!](http://actually.tryitonline.net/#code=NVDPhDtyO-KImWBNJ0FPaStjYE3ilapr4pmCzqNp&input=)

-4 bytes from Leaky Nun, and the inspiration for 3 more

Explanation:

    5Pτ;r;∙`MúEû`M╪k♂Σi
    5Pτ;                 two copies of 26
        r;               consume a copy for range(26), duplicate that
          ∙              Cartesian product
           `MúEû`M       for each (x,y) pair:
            M              maximum
             úEû           element in lowercase English alphabet, convert to uppercase
                  ╪k     list of length-26 sublists
                    ♂Σ   concatenate strings in each sublist
                      i  flatten and implictly print the stack