Well DNS servers needs to stay updated, fast and stable so it can quickly connect you to the closest server your requested site is hosted on.
Most ISP DNS aren't that stable nor are they fast. So they tend to be very unreliable at times, they won't disconnect you but response would be slow making you feel your internet is slow to connect to certain websites.
If you really aren't sure which DNS you should use and have no preference of using one over another then run DNSBenchmark  and see which is the fastest for you. I think you can add custom DNS servers to it too to check how fast they are, so you can try MTNL's DNS and compare with rest.