Herring Cove faces the right way, which on this peninsula is rare, and the evening ritual here is a genuine local institution rather than a thing the tourist board invented.
Come about ninety minutes before sundown. Park in the main lot, which empties in the late afternoon as the day crowd leaves and refills with the evening one. Bring a chair, a jumper and something hot — the temperature falls fast on the bay once the sun is off the water, and the wind usually swings.
The best light is not at the moment the sun touches the horizon. It is in the twenty minutes afterwards, when the whole western sky goes and the wet sand mirrors it. Half the beach leaves at the wrong moment every single evening.
Two practical notes. The fee booth is usually unstaffed by then, but the lot gates and the shuttle have last times. And if you want the sunset without the crowd, walk fifteen minutes north along the sand; almost nobody does, and the view is identical.
For the record, Race Point next door also faces water at sunset, but Herring Cove is the one with the flat walk from the car and the parking spaces pointed at the horizon.