Still the United States, due to the more direct election of the president. Cameron’s Party got just 36% of the vote last year. Hardly a mandate. Presidents usually receive around 50%.
The problem with direct election of the executive is that, in a two-party system like the United States, the parties are engaged in a zero-sum game of trying to screw each other, and there are almost never incentives to cooperate any more. So if you have divided government, nothing ever gets done.
The Conservatives getting 36% of the vote but 50+% of the seats in parliament is a problem, I totally agree, but it would be best solved by keeping the parliamentary system but getting proportional representation - then Cameron would have to take his 36% and pool it with one or more parties and share power, working together, and more accurately reflecting the will of the British people overall.