Pretty sure the Iraq War was the biggest stain on US credibility. Also there's a strong argument that the US support of Israel's occupation of Palestine (and by that I mean Israeli rightwing government, not the many good and courageous Israelis who fight for peace) is the next biggest stain on US credibility in international relations. It's been the single most consistently pointed to example of US hypocrisy that has run for decades. Consistently pointed to by radical Islamic preachers who wise to push people towards violence. Saudi of course is a closely followed third.