There are three different rarities of each Survivor you can recruit. These are:
As the name suggests, Common Survivors are easier to find and you can expect to recruit them on lower difficulty missions. They have basic stats and abilities.
Epic Survivors are more difficult to locate and have greater stats and abilities.
Supreme Survivors are the most difficult to locate and have superior stats and abilities.
You can purchase Survivors from the Black Market, including supreme types, so keep your eyes peeled!