In reply to The Lemming:
Each will be viewed through the prism of Brexit.
I think that Cameron will be viewed as a political lightweight, a PR man. He no doubt saw the pledge to hold a referendum on Brexit as an essential part of the job of stemming the flow of votes from the right of the party to UKIP. However, I don't think it was a vote that he thought he would lose. Perhaps he didn't appreciate the impact of his austerity policies, perhaps he underestimated the weakness of his campaigning or the strength of tactics (admittedly sometimes illegal tactics) of the Leave campaign.
Not thinking he could lose was ultimately wrong and a sign of arrogance, which meant that he did not plan for losing, which left a huge question mark after the referendum. A Brexit that wouldn't create massive divisions in society was possibly still possible then, but it needed someone to take the reigns and ultimately he ran away from the challenge.
May was desperate to be Prime Minister and secure her place in history. Her fatal mistake was pushing through the Article 50 agreement without proper dialogue will all of the parties (including her own) that had said that they would deliver the outcome of the referendum. She strikes me as a very poor communicator and as Ken Clarke described her a bloody difficult woman, but not in a positive way. This opened a door for the ERG and Farage to define what Brexit was, that being no deal rather than one of range of options with SM and CU arrangements at one end and WTO and no deal at the other, dismissing the softer options without serious consultation and rightly seeing WTO / no deal as catastrophic, ultimately gave her a position that couldn't be supported by a majority in parliament. Her days were numbered from this point.
The story hasn't played out with Johnson yet of course, he will fail if he does not deliver no deal Brexit by the end of the year; if he does deliver that, the impact on the country will be catastrophic and I suspect that his longer term legacy will be even worse, particularly if it results in a partial break-up of the union. I suspect that either outcome will result in a very severe weakening of the Conservative Party. I also think that history will roundly condemn his attempts to subvert democracy through the use of the prorogue.
Edit: The chances of Johnson coming to an agreement with the EU are so vanishingly slim that I have not even considered it an option.
Post edited at 16:37