There is no "best calling card service"

There is no "best calling card service" in general because it really depends on where you going to use it, when you use it and many other factors. Different phone cards work differently for different countries and may even work differently depending on time of day / week / holiday etc. For example, the same phone card can provide an excellent quality for calls within North America, to Europe, China, but terrible connection to Iran and not to connect to Somalia at certain time of day.

The reason is that phone card providers have to purchase "routes" to different courtiers from different providers and quality may vary depending on the route, country, route load at the time and many other factors.

In general, some providers and phone cards are doing better than others, but not entirely. For example, Canada's most well known phone card – CiCi. CiCi service provider – Group of GoldLine is Canadian largest provider of phone cards and other long distance services. GoldLine owns many key telecommunications elements and able to provide very competitive rates with quality to many destinations. However some smaller companies could get a better route with better price for some specific destinations, still providing poor service for other countries.

Recommendations:
The simple solution is to rely on a reputable service providers like CiCi phone card, Bell phone cards, etc. However, if you make frequent overseas calls you may get a good quality with better price from other phone card providers. For this you have to try different phone cards or take an advise from people who calls the same country. Remember, different phone cards work differently for different countries.