there is no such thing an “unbiased”; with that said, considering it’s based in the West and was founded by a bunch of World Bank officials, I would be extremely skeptical, and you can see numerous controversies listed in the Wikifedia article, e.g.:

It develops a Corruption Perception Index (CPI) based on surveys conducted by private structures or other NGOs: the Economist Intelligence Unit, backed by the British liberal weekly newspaper The Economist, the American neoconservative organization Freedom House, the World Economic Forum, or large corporations. (…) The IPC ignores corruption cases that concern the business world"