On which hill do the Ringwraiths ambush the hobbits?

The Ringwraiths ambush the hobbits on Weathertop, the common name for the hill crowned by the ruins of Amon Sûl. The attack occurs during Frodo's journey from Bree toward Rivendell.

Weathertop lies in Eriador, east of Bree and west of Rivendell. It was once an important stronghold of Arnor, the northern kingdom founded by Elendil, but the kingdom eventually fell and the tower became a ruin. Its height gives the travellers a useful vantage point, but it also makes them visible to their enemies.

Frodo, Sam, Merry, and Pippin are travelling with Aragorn when the Nazgûl find them. Aragorn tries to hold the Ringwraiths back with fire, but the Witch-king reaches Frodo and wounds him with a Morgul-blade. The injury threatens to turn Frodo into a wraith before he can reach Rivendell.

Weathertop is often confused with Amon Hen or Amon Lhaw, the hills near the Falls of Rauros much farther south. Those locations belong to a later part of the Fellowship's journey, while the Weathertop attack happens early in the quest.