LIGO made the first direct detection of gravitational waves in 2015.
The Laser Interferometer Gravitational-Wave Observatory, usually called LIGO, operates two large detectors in the United States: one in Hanford, Washington, and another in Livingston, Louisiana. The detectors measure tiny changes in the lengths of perpendicular arms caused by passing gravitational waves.
The first signal, later named GW150914, came from the merger of two black holes. The event was observed on 14 September 2015 and announced publicly in February 2016. It provided the first direct evidence of gravitational waves and the first observed merger of two black holes.
Virgo and KAGRA later joined the worldwide detector network, improving sky localization and confidence in detections. LIGO is therefore the answer to the first-detection question, even though modern gravitational-wave astronomy relies on international cooperation.