The most obvious impartial source for that would seem to be Amnesty International, but I can’t seem to find anything like that on their website.
Amnest Intentational specifically refused to get involved in ‘ranking’ nations on human rights, or in ‘ranking’ one type of torture as worse than another, etc. Their position is that every single human rights violation is wrong, and should be opposed.
Just because a government is generally good in recognizing the human rights of their citizens doesn’t give them a pass for a few cases of abuse. Their core vision (on the website you cited) is that "every person’ should enjoy all their human rights. Their emphasis is on individual persons, not dealing with governments.
I think they are concerned that any ‘ranking’ of nations would be used in political fights between nations, ‘my country is better than your country, nyah, nyah’; distracting the attention from the individual prisoners involved, in either country.
The only actual ranking that I found was on freedom of the press.
There is also an Observer Human Rights Index but the article seems to be from 1999.
The Freedom in the World 2006 index scores countries on civil libertries and political rights but there does not seem to be a ranking list.