Which would be a better choice- BITS Pilani Dual Degree with Mathematics, CS at DTU(DCE), CS at NSIT, CS at IIIT-Delhi?

The best option among the options you mentioned is BITS Pilani- Dual degree with Maths. This is because, with a good CGPA, you can even get into Computer Science there. Moreover, Mathematics is the foundation of Computer Science and all major coding problems. This excellent combination will help you open multiple doors for higher education. 

BITS Pilani is one of the best engineering colleges in India. It’s known for its excellent curriculum, faculty, lab facilities, and amazing campus. Moreover, BITS Pilani follows the GPA system which puts you at a certain level of advantage for pursuing higher studies, especially at foreign universities. 

The only major advantage of BITS is its location and high fees. Students from Delhi might find it difficult to adjust here. So unless you have no problem with the location and financial constraints, choose BITS. Else, NITS is also a good option.