Poor Corporate Websites
I would nominate British Gas as pretty high up that list.
It takes quite a while to respond, a long time to fully load the opening page (including filling the 'remember me' box), and then all too often won't let you open your account "due to a problem".
On the odd occasions when you can proceed further, you will be lucky if all the various features you want to use are available on the same visit.
Any others you know?