USB is probably the best but it has downsides. It uses up a precious usb port that you could use for something else. Also the cables cost a lot of money.
I'd say use parallel unless you have some high end printer that needs fast data transfer. Almost nobody uses parallel anymore, chances are far greater that you will need a free USB port before you need a free parallel.
If you run out of USB do to a printer, might need a new mobo. mine came with 12 total USB slots avalible. and most mob's have add-on for front USB or the extra that fit in a PCI bracket, and they plug right into mobo