Shaking a cocktail with ice serves two functions: you're chilling the beverage, but you're also melting a little ice into it, diluting the alcohol and rounding out the flavours. It's an important job, and a good shaker should be easy to fill, open, and strain, and it should do it all without leaking.
Photo by Didriks.
To find the best shaker for the at-home mixologist, Cook's Illustrated tested a whole bunch of different makes and models, making three different types of cocktails while tracking temperature, dilution volume, and leakage. The winning shaker from Tovolo formed a consistently "tight, and leakproof seal," was easy to open and close, easy to hold, and had a citrus reamer built right into the shaker, making the whole cocktail making process that much more streamlined.
Testing Cocktail Shakers [Cook's Illustrated]