First of all, I am constantly using four different SIMs of three different operators. I have Airtel, Robi, Grameenphone and Teletalk. I also have a Banglalink SIM which I use rarely.
Over the past few years, I have found that Airtel/Robi is the best for internet use in Bangladesh. I can get 4G internet service throughout the whole country. I have visited Saint Martins island last year where I got 4G on my airtel. This year I visited Sajek where internet connection is very weak for all mobile operators. Teletalk and Robi were there but it was hardly any good.
GP 4G is good but they have coverage problems in the whole country. I cannot tell about the Banglalink as I have rarely used it.
Teletalk has the cheapest voice and internet packages available but their network is terrible. If you have good network availability in your area, then you are lucky. Otherwise, teletalk is not a good option. I have faced numerous call drops on teletalk. Yet I keep a teletalk SIM for voice calling with my family. Because I use Bornomala SIM, so does my wife and my mother got a Oporajita SIM. Teletalk has the lowest call rate in Bangladesh. And in Bornomala you will get 30 minutes to use on teletalk numbers each time you recharge 30 TK. I find this very handy.
Overall, Airtel and Robi are best for internet use as they have maximum 4G coverage in Bangladesh. If you travel often, then Airtel and Robi should be your companion. If you live in an area when Teletalk has good coverage then you should give Teletalk a go.