The Taj Mahal changes with the light. At first light the marble glows soft and pale, and the crowds are at their thinnest. It is the visit most travellers remember.
To be at the gate when it opens, it helps to sleep in Agra. This journey takes you from Delhi in the afternoon, shows you the city at sunset, and puts you at the Taj at dawn, with a guide who knows where the light falls and a car waiting afterwards.
If you would rather do it in a single day, an extremely early departure from Delhi is possible. We usually recommend the overnight, and will explain the trade-offs when we plan it with you.