In order to select the best metal adhesive, you will need to consider a number of factors involving the job you are trying to accomplish. If your application calls for an adhesive that meets any specific qualifications or certifications, that is the most important factor to consider. You should also take into account whether or not your adhesive needs to bear a load, or meet structural requirements, in addition to how durable it will need to be. Epoxies are typically a good choice when you need to attach metal to metal, though they will not cure properly in certain conditions. Other adhesives can be well suited to specific situations, such as cyanoacrylates that cure in the presence of moisture, and anaerobic compounds that harden when they are not in contact with air.
There are many different compounds that can be used to bond metal to various materials, including glass, plastics, and other metals, so you need to consider your specific situation when looking for the best metal adhesive. If the job you are trying to do is subject to any rules or regulations, you may need to find a metal adhesive that meets specific qualifications, or is certified for that use. Some examples include adhesives that are proven to not break down in the presence of various gasses, or are certified for applications that involve potable water.
It is also essential to consider the materials you are trying to bond, and whether the adhesive needs to bear a load, when making your selection. Epoxies are often capable of bearing loads, though they are not suitable for every application. Two part epoxies typically require a long time to cure, so they can be a good choice in situations where it is possible to leave your components supported and undisturbed for an extended period of time. Another option is one part epoxies, though they are only suitable for applications where you can place your components in an oven, or heat them up through other means.
Various situations can each call for different adhesives, depending on the environment and other factors. If the components you need to attach are exposed to moisture, then a cyanoacrylate metal adhesive can be a good choice. Some adhesives will not properly cure if they are subjected to moisture, but cyanoacrylates actually require moisture in order to harden. Other adhesives, such as silicone, are flexible and heat tolerant, but not very strong. In some cases, combining this type of metal adhesive with screws or bolts to provide structural support may be your best option.