Exploits
Sometimes the tester may come across services that can be exploited. An exploit may be the means of initial access; just be sure that the exploit is 100% reliable. Also, running an exploit multiple times may crash the system. This option for initial access is typically used with extreme care, unless you have tested it and know what you are doing.
It's always SSH! Maybe it's not always, but I have never seen/can remember another service being used, outside of telnet, which should not be used anyways. SSH goes with Linux like peanut butter goes with jelly.