Qutb complex in New Delhi isn't actually just one attraction, but rather a whole collection of them. The site contains so much information on the history and culture of the Indian people that a tourist to India with an interest in learning more about that sort of thing could spend an entire day here and still not see everything.
The most famous attraction within the complex is definitely the Qutub Minar, a towering structure of brick that has found its way onto UNESCO’s World Heritage list. There’s also the Alai Minar, an uncompleted structure that would originally have dwarfed the Qutub Minar in size, and the Quwwat-ul-Islam Mosque, which dates back actually to the 12th century and features all kinds of impressive architectural feats such as archways and carvings that are a must see. Make sure to stop in for a look at the Iron Pillar as well, one of the most impressive metal structures of the ancient world.