1- Winston Churchill, 2- William Gladstone, 3- Margaret Thatcher, 4- David Lloyd George, 5- Lord Salisbury, 6- Viscount Palmerston
7- Clement Attlee, 8- Robert Peel, 9- Earl Grey, 10- Benjamin Disraeli, 11- Henry Asquith, 12- Stanley Baldwin
13- James Callaghan, 14- Harold Macmillan, 15- Earl of Derby, 16- John Major, 17- Henry Campbell Bannerman, 18- Lord John Russell
19- Tony Blair, 20- Alec Douglas Home, 21- Andrew Bonar Law, 22- Harold Wilson, 23- Ramsay MacDonald, 24- Arthur Balfour
25- Lord Melbourne, 26- Edward Heath, 27- Earl of Aberdeen, 28- Lord Rosebery, 29- Neville Chamberlain, 30- Anthony Eden.
Obviously the great leader Gordon Brown can't yet be considered for this list until he completes his term but I think he might be in the top 30. Also for a few of the more obscure PM's my judgement on them is based on very few sources.
* Actually arbitrary is probably the wrong word, subjective is more like it.