@OP
What SIM you using now ? Because if there is a decent plan in your operator why to change it (unless you plan to port number to an operator with better internet plans). I find 3G ridiculously expensive so won't talk about that. For 2G here are my takes. Not in any order of preference
1. Airtel
Plan - 2GB/98 Rupees per month
Decent and consistent speeds, wide connectivity.
2. Reliance
Plan - 3GB/99 Rupees per month (please confirm)
Very good speeds. I have had constant DL speeds of 30+ kBps
3. Tata docomo
Plan - Various (1GB for 21 days for 48 rupees, 2GB/65 rs/30 days and so on)
Poor tata docomo connectivity (in many areas, not all). Use it in 3G mode to get consistent 128kbps speeds(it is capped). This is better than GSM mode since speeds in EDGE will keep on shooting up and down.
4. Vodafone
Plan - 99 rupees for 2GB or 3G data (dont remember)
Good speeds and wider network.
Others never tried.