My answers are always based on my own experience not on rating or review done by any other crypto gurus or experts., here also I am writing this answer from my own experience.
There are three major exchanges I have invested through.
Zebpay ,coinsecure and Unocoin.
Zebpay has super mobile app and fast customer support. If you transfer your money to Zebpay, within 6 working hours your money will be available in your INR wallet . BTC purchase is real time . Few people say that price of BTC in Zebpay is slightly higher than other exchange but I did not notice any substantial difference.
Coinsecure is a web based bitcoin exchange, they have no mobile app. Their support due to limited IT infrastructure , is not as prompt as Zebpay but they have plan to expand the infrastructure and support shortly. But they are trustworthy.
Unocoin is the worst exchange . They have no customer support, even CEO is not responsive in social media.
But if have no reservation to buy bitcoins from an international platform ,you can tryl In you can compare the prices of bitcoin before you purchase, so if you are lucky you can get bitcoins at very lucrative price.
Changelly is also a very good platform to exchange your bitcoins with etehrum or some other cryptocurrency.