Rwanda, everyone acknowledges that one.
Other genocides which America could but didn't intervene or even encouraged:
Zair 1996
Iraqi Kurds late 1980's
Cambodia late 1970's
The South American Death Camps 1970's
East Timor 1974
Cyprus 1974
Bangladesh 1971
India dealt with Bangladesh, and Vietnam with Pol Pot. Of course at the time of those events, the US was pro-Pakistan (India was too "pro-Communist", you see) and then, in one of the more absurd and shameful moments of post-war diplomacy, condemned the Vietnamese invasion and proceeded to give support to an "opposition" containing "moderate <sic> Khmer Rouge".