AWK POSIX – AWK/NAWK Alternative on SunOS and Linux

I don't see why you would want to do it in a single awk command, what you have seems perfectly fine. Anyway, here's one way:

$ awk -F, '(max[$18]<$21 || max[$18]==""){max[$18]=$21;line[$18]=$0}
            END{for(key in line){print line[key]}}' file
6598,6598,0,1,,1,0,1,1,0,0,0,1,0,0,0,0,1390,1390,,0.730000,
1297,1297,0,0,,0,0,1,0,0,0,0,0,1,0,1,0,1707,1707,,7.000000,
6553,6553,0,1,,1,0,1,1,0,0,0,0,1,0,1,0,4326,4326,,9.000000,

The idea is very simple. We have two arrays, max has $18 as a key and $21 as a value. For every line, if the saved value for $18 is smaller than $21 or if there is no value stored for $18, then we store the current line ($0) as the value for $18 in array line. Finally, in the END{} block, we print array line.

Note that the script above treats $18 as a string. Therefore, 001 and 1 will be considered different strings.

if awk --version 2>&1 | grep -q "GNU Awk"
then
    awk 'BEGIN {print "I am GNU Awk"}'

elif awk -Wv 2>&1 | grep -q "mawk"
then
    awk 'BEGIN {print "I am mawk"}'

else
    awk 'BEGIN {print "I might be nawk, might not be"}'
fi

Alternatively, test is awk is a symbolic link:

awk=$( command -v awk )
[[ -L $awk ]] && readlink $awk # make some decision about the result of that