The simplest way to cook shrimp, for more flavor leave the head and shell on.  If you must peel them save the shells for stock or flavored butter.

Cook the shrimp in a court bouillon or highly seasoned stock that is at a boil. Place the shrimp in the boiling liquid bring back to a simmer.  Twenty-Twenty five count shrimp will take about two minutes after stock returns to a boil.  Of course larger will take longer while smaller requires less time.