I'd have to say PCI. There aren't many PCI-E 1x devices available, and it seems like the few that are available are unnecessarily expensive.
2nd choice would probably be parallel and serial, I still have a few devices that use those.
And FFS, replace that crappy floppy drive with a USB stick! But I haven't had to boot from a thumb drive in a while because it seems like all boards nowadays can be flashed from Windows.
