AltSoftSerial is the best software serial library. Only one instance is allowed, and it is must be used on one of the Input Capture pins (pins 8 & 9 for a UNO, Mini or Nano, see docs here).
My NeoSWSerial is next best. It works on any two pins, but only at baud rates 9600, 19200 and 38400.
SoftwareSerial is the worst choice. It works on any two pins, but it is very inefficient. It disables interrupts for the entire time that a character is being sent OR received, and it cannot do both at the same time (unlike all the other serial choices). This can interfere with other parts of your sketch, other device communications, or with libraries.