From my personal experience, I would suggest you to join Vajiram and Ravi for IAS coaching. This is the best place to prepare for IAS exam. Be it the experience and support of teachers, quality of study material, location or test series I found everything up to the mark.
Though some might call it a bit expensive, but when you look at the quality of coaching, well-researched study material and the comfortable infrastructure, you will find it completely justified. The research team at the coaching center keeps a close eye on the changes in exam patterns and prepares test series accordingly to help students face any surprise in the examination hall. For me, Vajiram and Ravi is the best IAS coaching institute in Delhi.