In which city is the seat of the Steward of Gondor?

The seat of the Steward of Gondor is in the city of Minas Tirith.

Minas Tirith is Gondor's capital and a seven-level fortified city built against Mount Mindolluin in the White Mountains. Its highest level is the Citadel, where the Tower of Ecthelion contains the throne room and the central offices of Gondor's rulers. The Stewards governed from there after the line of kings ended.

The last King of Gondor, Eärnur, died in T.A. 2050 without an heir. Mardil became the first Ruling Steward, and his successors administered Gondor from Minas Tirith for nearly a thousand years. Denethor II is the Steward seen during the War of the Ring.

Osgiliath was once Gondor's principal capital, but the royal court moved to Minas Anor—later renamed Minas Tirith—after Osgiliath suffered war and plague. Dol Amroth and Pelargir were important Gondorian cities, yet neither was the seat of the Steward. After Aragorn's return, Minas Tirith again became the capital of the reunited kingdom.