If you want use laptop at home thane Reliance Broadband is one the best for you. They have best speed and very cheap plans. I am using it since Oct. 2005 and I have no problems. Check their plans: http://www.reliancebroadband.co.in:80/pages/flash_banner/infopage.html If you want for anywhere used than Airtel GPRS is good.
Well GPRS servcies of different Mobile service provider is best but almost all are slower. You should check some wireless service provider in your area. You can also use Relince Data card. You can enjoy it 1500Rs per month and 1.5 GB use. Speed is upto 154 KBPS but I got constant 75 kBPS to 80 KBPS which is more better than GPRS.
The Best Internet Service Providers in India for me are Airtel and BSNL BSNL sadly doesn't come in Mumbai