There's a thread about the worst plausible US president, so thought I would do about British politics.
Who would in you opinion be the worst PLAUSIBLE British Prime Minister. The stress here is on plausible, so the person must be someone who was considered to be a significant contender for the position and who has at least a moderate level of support in sections of the population. In the British context, this basically means that they must at some stage have been a cabinet or shadow cabinet minister.
Please give reasons for your choice.