Mathematica, 69 65 57 bytes

^{Saved 8 bytes due to @MartinEnder.}

FromCharacterCode[64+Max~Array~{26,26}]~StringRiffle~"
"&

Anonymous function. Takes no input and returns a string as output. Basically just takes char('A' + max(x, y)) for all x, y from 1 to 26.

Wolfram Language (Mathematica), 69 65 57 44 bytes

^{Saved 8 bytes thanks to Martin Ender.
Saved 13 bytes thanks to att.}

Print@@@Array[Alphabet[][[Max@##]]&,{26,26}]

Try it online! Full program which prints to standard output.

Mathematica, 69 65 bytes

StringRiffle[FromCharacterCode[64+Max@##]&~Array~{26,26},"
",""]&

Anonymous function. Takes no input and returns a string as output. Basically just takes char('A' + max(x, y)) for all x, y from 1 to 26.

Mathematica, 69 65 57 bytes

^{Saved 8 bytes due to @MartinEnder.}

FromCharacterCode[64+Max~Array~{26,26}]~StringRiffle~"
"&

Anonymous function. Takes no input and returns a string as output. Basically just takes char('A' + max(x, y)) for all x, y from 1 to 26.

Mathematica, 69 bytes

StringRiffle[ToUpperCase@Alphabet[][[Max@##]]&~Array~{26,26},"
",""]&

Anonymous function. Takes no input and returns a string as output. Basically just takes letters[max(x, y)] for all x, y from 1 to 26.

Mathematica, 69 65 bytes

StringRiffle[FromCharacterCode[64+Max@##]&~Array~{26,26},"
",""]&

Anonymous function. Takes no input and returns a string as output. Basically just takes char('A' + max(x, y)) for all x, y from 1 to 26.