Hard to choose. They've all got good and bad points. Churchill not only got us through the war like a champion, but back when he was a Liberal he worked with Lloyd George to lay the foundations for the welfare state too, something that's often overlooked these days. On the other hand, his record on the subject peoples of the Empire was pretty abysmal.
Blair had a lot of faults, certainly, but he made the Labour party electable again. Minimum wage, civil partnerships and the freedom of information act were great too, even to the extent that Blair now regrets the FOI. MilliD is very right to point out that there's much to be proud of in Labour's recent record. Student fees has been a shambles though, and the ever expanding number of graduate places on pointless courses doesn't help when the ones who are coming out can't get jobs. And it's hard to ignore his role in the ballooning public spending of the last ten years, that still failed to make a dent in equality, that didn't find jobs for the million and a half that were out of work for the entirety of the last decade, and yet has seen armed forces budgets cut to the bone while we fought a succession of wars. ID cards, rendition, 42 days detention...
Major was a bit tim-nice-but-dim, not particularly harmful but certainly not great either. It's a terrible indictment of Kinnock that he managed to lose to him in 92, especially considering the effect of the 90's recession.
As for Attlee, all substance and no show, a man of his enormous ability would have been a credit to any government at any time, but he's certainly the finest Labour PM we ever had. But even he's got his flaws, his governments failure to produce a workable solution in the mandate is the root of a lot of modern middle eastern conflict.
And I personally think quite well of the Iron Lady, as do my parents. My mom has voted for the Liberals all her life in one variant or another, and my dad for Labour, but they both switched to Thatcher. There was a period of time where the unions had the country on its knees and my parents suffered badly. And no, they weren't rich, or even middle class, far from it, but when the unions are all at home on strike pay then there's not much work for anyone else either.
They've all had their good and bad points, and I find it quite hard to choose. The thing I've come away with most from this little thought exercise is how fortunate we've been to have had such an extraordinary collection of extraordinary talents at the helm.