There is no single best deep socket set, because different applications can require a variety of features. Sockets come in four possible drive sizes, so you need to choose one that is large enough to provide the necessary torque but not too big to fit in your work area. Not all sets have the same depth either, so it is important to choose one that provides enough space within the sockets to accommodate the studs that you are likely to encounter. Other options such as six- or 12-pointed sockets, or a set that includes built-in universal joints, can also be the best choice for certain situations. You should also choose an impact deep socket set if you will be using an impact wrench or air gun instead of a hand-operated ratchet wrench.
The first factor to consider when choosing the best deep socket set is the drive size. There are four common drive sizes that range from a quarter inch to three-quarters of an inch, which are international standards that are used even in countries that employ the metric system. The larger sizes add both bulk and the capability to handle higher levels of torque, so people who often work with tools will typically own sets in at least two drive sizes. If you will be working in very tight quarters and are unlikely to encounter bolts that are especially tight, a quarter-inch drive can be the best choice. In many common cases a three-eighths drive can provide both torque and accessibility, and larger sizes are usually used for applications where substantial tightening or loosening power is needed.
Another factor to consider is the actual depth of the sockets, because this interior space is required when tightening or loosening bolts that are attached to studs. Some deep socket sets provide less interior space than others, especially if the material they are made out of is inferior or weak. If you examine the interior space of a deep socket, it should extend for a significant portion of the socket length. If it does not, that particular deep socket set will not provide much more utility than a normal shallow socket set. Some manufacturers also offer sets that are halfway between shallow and deep socket lengths, which can be the best choice for some jobs.
The number of points that the sockets have can also be important. A six-pointed pattern typically provides more gripping strength and torque, but a 12-point socket can be used to adjust square nuts in addition to the more common hexagonal variety. Some six-pointed sockets also include proprietary designs that can be useful when dealing with damaged nuts, which can be the best set to choose if you are likely to encounter fasteners that have been removed or adjusted previously.
One major distinction that can be made between socket sets is whether they are designed for impact use. If you will be using your deep socket set in conjunction with an impact tool, you should make your choice based on that fact. Many sockets that are not designed for impact use are made of steel that is both strong and brittle. This is fine for most applications, but these sockets can shatter if they are used with an impact tool. Sockets that are designed for impact use typically have a black oxide coating instead of chrome or another decorative finish.