What is the best calling card to call Nepal from USA?

After a long gap I saw a query on calling card. I use Ribtel. 

i use amantel costs aron 10 cents a min.

viber outcall/out dial (not Viber to viber which is free)... costs around 8.5 cents... the connection is as if you are directly calling from your cell phone to another cell phone.
With Viber out call, you can call any number in Nepal. The other number does not need to be connected to viber at all.
The only caveat is, you have to be in internet and use your viber app.

wish there would be alternatives like a cent per minute. I see so many ads that other countries can be called from the USA like a cent per minutes and so cheap to call unlimited.

I use Keku, www.keku.com

it has good referral programs, and bonus minutes. it's 9.5¢/min ~ 9.9¢/min

I use mobile VOIP which is i think about 19cent per minute. Viber also has outcalling with similar cheaper rates. With data being widely available in nepal I use the phone as a backup and mostly use viber / whatsapp and tell my family to call back. Calling back from nepal is a lot cheaper than calling from here.

Use Google Voice . They have 10 cents/Min . Download google voice app and start using it. Also get a free number.
https://www.google.com/googlevoice/about.html