Perhaps a good corallary question is which system has the most egregious flaws to it. The concept of "government" incorporates more than election methods; one of the biggest problems of government is the ideologic battles that rage and the dirty statesmanship that is practiced. It seems that whichever government has the soundest system for suprressing this sort of hatred politics is the government with the best system--and not in a fascist sort or way--rather in an honorable way of honest debate and compromise with the will and wellfare of the people made more important than party ambition and special interest power.
Just to look at this issue from a different view point other than my own, which would naturally support Britain as having the best system of law, Germany definitely does have an electoral system that reflects the wishes of its electorate better. With First Past the Post, it is possible to have more individual votes cast nationwide for one party but for another party to win the election. Also, the Proportional Representation System gives smaller parties more of a say, which they don't have in two-party America or three-party Britain.
This is, of course, purely a matter of opinion. These three countries are all democratic and all have a strong commitment to upholding the rights of their citizens. None of them is perfect, but all are amoung the freest and most democratic countries in the world (as shown in the map in the link below).
There are differences between the three nations. Only the US has a system with full separation of powers between the three branches of government. This can be seen as a more democratic system of government. On the other hand, Germany is the country that has the system that is closest to providing proportional representation for voters of all parties. There is no objective way to say whether a system with separation of powers is better than a parliamentary system or that a "first past the post" system is better than a proportional representation system.
So, all of these are free countries and it is impossible to objectively pick which is "best."