The Mosteiro dos Jerónimos
We've only been here for a couple days, so it's hard to be definitive, but one of the most unforgettable sights in Lisbon must surely be the Jerónimos Monastery, in the western neighborhood of Belém. Construction began in 1501, during the height of Portuguese power, and the complex has remained in incredible condition. UNESCO inscribed it as a World Heritage Site in 1983.