This is most important tool. Sure, you can shake drinks in other things, but if you want to make a real drink, you'll need one of these. The brilliance of the Boston Shaker is that it works with any standard-sized pint glass. That glass also serves well for stirring, which, for moderate use, is an acceptable stand-in for a dedicated stirring glass.
While the Cobbler Shaker is probably the most commonly used shaker because it has a built in strainer and fitted top, bartenders prefer the larger Boston Shaker style to better chill drinks.