God knows off the top off my head. There were all sorts over the 14 years and I don't keep a ranked list in my head to spout on demand. Truss not getting the BoE or markets on board before the radical budget, was certainly one. And in that budget, reducing the top rate of tax from 45% to 40% whilst making economic sense, was unnecessary and optically really bad. Not getting immigration under control, definitely another. Partygate was not a big deal in my mind but I can see how politically damaging it was.
How about you? What do you think were their biggest blunders?