Which is the best institute in India for IIT entrance exam preparation?

There are many good institutes offering coaching for IIT-JEE. You can look for Aakash, Resonance, Allen, Bansal classes, Catalyser. Kota is the best place to prepare for IIT. Check the below link for more- https://www.shiksha.com/b-tech/articles/iit-coaching-in-kota-know-the-best-institutes-to-prepa

Fiitjee- Kalu Sarai and Punjabi Bagh centres which are in Delhi are really best for IIT rest it depends on your location, you might find a good institute nearby your locality.