Fatehpur Sikri might be my favorite place in India. The city was built in the late 1500s, and was once the capitol of the Mughal empire. Although the city itself doesn't contain anything very interesting or noteworthy, the cluster of buildings on top of the mountain is a very important stop for religious pilgrims - the Jama Masjid is one of the largest mosques in India.
There are three things of interest on top of the mountain - the mosque, the old royal palace, and an abandoned cluster of buildings behind the mosque. The royal palace was extremely special to me because it was so unexpected - I have never seen such carefully tended grounds anywhere in India, or indeed on most of my travels. Since Fatehpur Sikri is in the middle of pastoral nowhere, there weren't too many tourists, and it really felt like stumbling upon a secret garden.
See for yourself!