The best double boiler is one that is large enough for your needs. If the bottom pot that holds boiling water to heat the top pan is too small, this can be messy or even dangerous. Water can boil over the brim and out onto the stove; if the bottom pot boils dry, a fire could start if it's left that way for too long. Choose a double boiler that includes a water fill line on the bottom pot as well as heating instructions for your safety. Many people mistakenly use double boilers on high heat rather than low to moderate stove top settings.
The steam from the boiling water in the bottom section provides the indirect heat to the top part. The French call the double boiler a bain marie, or "water bath." The indirect heat created by the two pan system melts chocolate without scorching it and heats milk without curdling it. Double boilers are also ideal for making cream and cheese-based sauces as well as sweet mixtures such as caramel.
The best double boilers to buy are the ones that feature the two pots noticeably different in size. If the top and bottom sections are too close in size, the upper pot may slip down into the lower one, defeating the whole purpose of a double saucepan system. If the lid included in a double boiler set fits both pots, this may be a clue that the pot sizes are too similar. Consider choosing one of the double boiler styles with a glass lid for the top pot. A glass lid on the upper section of the boiler can allow you to easily see when and if your food item is melting.
Quality double boilers often last for many years and most of these even come with a lifetime warranty. Buying a professional type of double boiler is the best option because the price difference between that and one made to a lesser standard is minimal. Good stainless steel double boilers are often labeled professional quality. While non-stick double boilers are available for sale, these are not usually considered to be professional quality cookware.
Look for a double boiler with handles that are easy to grip. You should also make sure the handle length works well with your stove top so your hand isn't too close to the heating element. Finally, be sure the double boilers you're choosing between fit in with your cleaning and maintenance expectations; not all are dishwasher safe.