There are two types of reliability people look for in a life insurance company.
First, you want to know whether you can trust the insurer to honor your agreement. Life insurers have a strong record of paying promptly and as promised. A LIMRA study from 2012 found that around 90% of people who had already experienced a positive claims process expressed confidence in life insurance companies.
Second, you want peace of mind that your insurer will remain solvent over the life of your policy. There are several different businesses that now track the financial health of insurers and issue credit ratings. Although these ratings can differ slightly with each rating company, it's a good way to quickly check for signs of trouble. (Specifically, if you ask an insurer for its rating and it fails to provide one or to direct you to one of these rating companies, you should take that as a sign of unreliability.)
Here are some popular sites for insurance company ratings: