Phone SIM card

What part of the country? Coverage varies, as does the provider with the most coverage.
My only experience with prepaid has been Verizon and t-mobile. Both have worked well, Verizon has more coverage and faster. For a truly "unlimited" deal you can expect to pay more but the basic prepaid plans typically include the first 1-5gigs of data and are around $50usd for a month. Ymmv.