There are hundreds of VoIP providers and everyone has its own strengths and weaknesses. My favorite and currently using is Onesuite.com which started as a prepaid phone card service but now added a voip feature in their services.
There are available reviews you can read over the internet. These reviews will help you choose a provider. From here, you can compare different providers.
I am using netTALK DUO now, and am quite happy with its performance. Check out its call plans: http://www.nettalk.com/callplans/