Here's one that's amazing, more for its history than anything else:
tr -cs A-Za-z '\n' |
tr A-Z a-z |
sort |
uniq -c |
sort -rn |
sed ${1}q
(source and details: leancrew.com/all-this/2011/12/more-shell-less-egg)
This is Doug McIlroy's six-command response to Donald Knuth's "10 pages of pascal" solution to the challenge: Read a file of text, determine the n most frequently used words, and print out a sorted list of those words along with their frequencies.
It was a cheeky response because Knuth was actually demonstrating 'literate programming', which was intentionally verbose; and even McIlroy admitted Knuth's solution was more refined/powerful/extensible. But it was a great way to show the power of unix pipelines.
Which goes back to my original thought for this thread - it's not the individual commands that are amazing, it's how those simple pieces combine that breaks your head ;)