Who is the sleepiest of them all?
Pyth, 12 10 bytes

hoSN%2.:z3


This prints the sleepiest emoticon. Verify all test cases at once  in the Pyth Compiler.

Credit goes to @Sp3000 for the idea to use sorting.

How it works

hoSN%2.:z3
              (implicit) Save the in z.
      .:z3    Compute all substrings of length 3.
    %2        Keep every seconds substring. This discards non-emoticons.
 o            Sort the emoticons by the following key:
  SN            Sort the characters of the emoticon.
                This works since '-' &lt; '=' &lt; 'o'.
h             Retrieve the first, minimal element.