The worst switches I've ever used would probably be various 'Cisco' switches from their small business line, usually ones that ran the same OS used when they were sold under different names like 3Com or Linksys.
oh god, when I worked for a small telecom in the midwest they heavily used the 3Com switches. They were the bane of my existance, things would power loop, or my favorite, continue to work but prevent any sort of access to them.