Calling a certain coaching center best is not a right way to answer your question for something that is great for me might not be equally appealing for you. For instance, you can like a coaching center that has good teachers but just average infrastructure, while for some other student both the qualification of the teacher and the quality of infrastructure might be equally important.
Since you want to get a name, I would say it is Vajiram and Ravi. This is a perfect coaching center for UPSC and IAS exam preparation. They offer well-researched study material, regular classes and test series.
Though you might find their fee that is around Rs. 1lakh a bit high, but when you compare it with the facilities provided, you will find it completely justified.