Find all files (-type f) and execute the command "cat" on each of them (+), then sort the output, count the number of occurrences of each line (-c) and sort the output by the number of occurrences (-k1,1rn) and display the top 5 lines (head -n 5).
find . -type f -exec cat {} + | sort | uniq -c | sort -k1,1rn | head -n 5