Re: what is the best prepaid phone and service?
This is a nebulous question, unfortunately. There is no right or wrong answer as it depends on what packages you need. Do you use more minutes of calls, or do you need large amounts of data, or are you a purely SMS person? Which country are you based in? No point in us telling you about offers in Senegal if you are based in Lichtenstein...
Assuming as you are on an Apple site that you are looking at an iPhone, you may wish to consider what network has best coverage in the areas you intend to use the phone. For instance, whilst 'Example Network A' might offer you a killer deal, if their coverage isn't up to scratch where you need it, it's pointless going with them and you'd be better off going with another provider.
Basically, the parameters are far too wide for us to give you any kind of meaningful comparison. You're best off checking the sites of the service providers in your area and seeing which ones give you the best bang for your buck.