please advise on buying a wifi pen drive (New Delhi)

I would like to be able to use wifi internet with my laptop in all parts of Delhi. Would you please recommend which company is the best provider and which pen drive to buy (best brand, speed parameters, etc) ?

I was about to buy the Airtel one, but at the same time another client walked in the Airtel shop and complained that his plan was not working in most places, so I dropped this one :) I've heard that Tata and Vodafone are also providing such services. Any specific suggestion ?

Many thanks in advance !

Olga