Definitely Delhi-Chennai Catering highest number of express train traffic on an average end to end. Plus this route caters branch line trains also like:
Mathura Jn- to Kota, Mumbai
Agra Cantt- to Jaipur
Gwalior- to Indore
Jhansi...
more... Jn- to Lucknow, Gorakhpur, Allahabad
Bina Jn- to Katni, Kota
Bhopal Jn- to Indore
Itarsi Jn- Jabalpur, Mumbai line
Nagpur Jn- Durg, Raipur, Surat Mumbai
Warangal Jn- to Hyderabad
Vijayawada Jn- to Guntakal, Vishakhapatnam
Gudur Jn- to Tirupti.
This route is approximately covering 60% of India.
Even beyond delhi and Chennai this route will benefit Entire North, South and Central India and partially East and West India too.