In my programs I usually only encounter 3 types of epic one liners, they all use the ternary conditional operator extensively.
(all in c++)
Type 1: Conditional arithmetic - When I want to change how an equation behaves based on conditions. Example taken directly from the add function of my integer library: (i<s?v[i]:0)+(i<o.s?o.v[i]:0)+(carry?1:0).
Type 2: Inline switch statements - Any switch statement can be made inline with the ternary operator.
Type 3: Complicated bitshifty stuff, typically to create inline hash functions. I can't think of a good example off hand, but they end up looking really cool.
However, I believe epic one liners are generally considered bad practice. That is why I try to keep a comment above each one that describes what the one-liner does.
get the factorial of integer n
fact = lambda n:[1,0][n>1] or fact(n-1)*n
# test it ...
''' result ...