PlusNet and AOL. It's a joint thing, because I only had an AOL free trial, so I didn't have them for long (temporary stopgap due to PlusNet issues). Had to contact AOL support once, and it was useless, and ended up fixing the problem myself.
PlusNet for traffic shaping when I don't think we actually reached their stupid "fair usage" cap and for generally being rubbish.
Also charged us extra when we left as we were on the 1-month contract thing, but they updated the T+C partway through (and didn't tell us) that there's a 12-month minimum. On the new contracts it actually says reasonably clearly there's a charge if you discontinue before 12 months, which really defeats the point of a 1-month renewing contract.
Person responsible for the internet bill at the house coughed up (I was willing to hold out).
Honourable mention goes to So Internet (previously known as WFCS) - used to have them at my old workplace and they were pretty useless. We had hosted mail with them as well, and they apparently did a manual transfer of user accounts on their mail hosting (without saying there was maintenance), and we only found out because they couldn't spell my surname ><.
