Margaret Thatcher was the best PM, because she not only managed the country well, but the set a precedent that will last for ever that a country can be governed using pragmatic and practical sense rather than shameful dogma.
I really don't understand why people rate churchill so highly. Gallipoli and the gold standard weren't the brightest of ideas. OK, so he wasn't PM at the time, but with the exception of rousing speeches what did he do as PM? I wouldn't say he was bad though, I just think there were better.
Thatcher. The fact that she took the unions on and won, preventing our decline into socialism, would have been enough, but her other achievements (teaching the Argentinians that Britain is not to be trifled with, the fact that she was the first woman to become PM, her sheer longevity in office) means she's completely unassailable. Although Churchill is a close second.