As per my experience Airtel Broadband is the best. Their down time is very less and also the customer care is very good. Speed of the connection is also good and you get value for your money with Airtel Broadband connection.
I really had a bad experience with BSNL which I used few years back. Their service is bad and the down time ranges to days. Their customer care is not at all reachable and they never resolve the issue as per the promised time or date.
I also used Hathway connection which I felt was much better than BSNL. Though you cannot compare it with Airtel still a reasonable one when compared to BSNL.