How do I get only the diagonal with awk?

awk'{print$NR}'aa.txt
 awk'{print$(NF-NR+1)}'aa.txt

NR is the built-in variable that substitutes the number (several lines from the top) from the beginning of the line being processed.

NF is an embedded variable that substitutes the number of fields (columns) in the row being processed.

$ is an operator that refers to a field and is often used as $1 (=first column) or $2 (=second column), but you can also give an expression in the form $(expression).(*The first code omitted ())

$NR...terrible

awk'{(i=NR%NF)?0:i=NF;print$i;}'aa.txt
awk'{(i=NR%NF)?i=NF-i+1:i=1;print$i;}'aa.txt