From best to worst:
1) Parallel add-on
2) Switchbox
3) USB to parallel adapter
My qualifications: HP Corporate Laserjet technical support
Reasons:
1) True parallel port, direct connection, ideal solution
2) Still a true parallel, some devices may have trouble with the switch but for the most part it's generally pretty safe. Will generally be supported to some degree or another.
3) Unsupported by the printer manufacturer, not a real parallel port (try doing a DOS dump to it

), requires drivers
Viper GTS