I have used paytm.com a lot of times to recharge my vodafone and airtel mobile phones.
The website is very user friendly and the transactions are always successful. The best thing is that they show the value that you will get with the amount of recharge. No other website gives that facility. I always recharge my Vodafone mobile for Rs. 410 (full talktime). The phone gets recharged immediately.
The website is very fast too, just takes 1 min for the whole process.