Glaswegians definitely. Thick as mince the lot of them and not even decent mince at that. Happy Shopper mince. The land that Time and Soap Forgot.
Brummies and West Country. I don't think South Wales is especially bad but North Wales certainly is. Isnit? There's nowt so think as a Snowdonia accent.
Probably Brummie, but least intelligible has to be something Scottish. Some of the Highlanders might as well be talking in Urdu.
Trouble with the Gogs is that I can't tell whether their speaking English, Welsh or Swahili half the time, though I would have said it sounds more stuck up their own arses than thick. Apart from Wrexham, they're just scousers with speech impediments.