Glad you agree about the war crimes, which in itself should be enough to rank him amongst the very worst PM's, as to the rest:
Buggering up devolution
We don't have a Scottish government dominated by nationalists pushing for a new referrendum every 5 years until the voters deliver the right result, and at the same time doing their level best to cause havoc at Westminster with the West Lothian question unanswered, that's good to know.
Setting the stage for our exit from the EU
I could write about immigration and the multicultural agenda and how this alienated a large part of the country, but I think the simpler option is just to point at the timeline, in 1997 Europe was a fringe issue of interest to about 30 MPs in the Conservative party, by 2015 it was an election defining issue, that wasn't Cameron and Clegg's work.
Crashing the economy
I know those on the left like to blame "the bankers", but who was it who introduced the "light touch" regulation system that allowed RBS and others to be so badly mismanaged and then create the 2007/08 financial crisis?
It was the government's job to regulate the financial sector, they failed.
I should mention that the four points I listed were not exclusive but simply the most obvious ones I could think of at the time, I could also mention PFI and some other fairly disasterous changes in the NHS, or perhaps the way in which he (and Brown) hollowed out the Labour party
and left them (after 13 years in government) with little talent at the top of the party and a series of increasingly duff leaders, but I've spent more than enough time on this already.