Where the 1945 Simla Conference and Partition-era talks happened - genuinely significant history, not just a pretty building. The interior is by guided tour only (about 30 min, capped at 60 people, Hindi and English, first around 10:30am, last 4:30pm, lunch break), so check slots on arrival. Photography inside is restricted; the grounds and gardens are worth the walk even if you skip the tour.
Viceregal Lodge (Rashtrapati Niwas) was built between 1880 and 1888 as the official residence of the Viceroy of India, commissioned under Lord Dufferin and designed by British architect Henry Irwin in Jacobethan/English Renaissance style reminiscent of Scottish highland castles. It was the first building in Shimla wired for electric lighting and had its own steam generator. The Lodge hosted the pivotal 1945 Simla Conference and discussions that led to the 1947 Partition of India; it now houses the Indian Institute of Advanced Study.