The answer to your question depends entirely upon what coverage you want, what data speed you want, and how many minutes you want.
Right now, here's what I've found:
Simple Mobile - $40 / month unlimited everything, very slow data speed, Tmobile coverage without Tmobile roaming partners. You have to bring your own GSM phone, and you'll want a 1700/2100 (i.e., Tmobile) phone if you want high speed data. However, their limits on HS data make it irrelevant. For $60 a month, they give real high speed data, but there are better options.
Walmart Family Mobile - $45 per month for first phone, $25 per month each additional line, data purchased pay as you go with rollover, Tmobile with roaming partners. I haven't heard that data speed is limited.
Straight Talk- $45 per month for unlimited everything, coverage depends upon your ZIP Code and which phone you choose. Can use At&T (with roaming partners), Tmobile (with roaming partners), or Verizon (with roaming partners) for non-android phones, depending upon ZIP code entered at purchase.
To find out, shop for a phone on their web-site. After entering your ZIP code, look in the URL (http:/) address in your browser. Look for the letters after "market=". If GSM4 appears in URL, all phones will be AT&T except Android Phones (which will be Sprint without roaming partners). If GSM5 appears, all phones will be Tmobile except for Nokia Smartphones that require the $45 unlimited plan (will use AT&T with roaming partners) and Android phones (Sprint, no roaming partners). If CO appears, phones will be Verizon CDMA except android phones.
You can force a particular type of phone by clicking shop, phones, and then entering one of these ZIP Codes.
AT&T- use 95521
NOTE: Android phones operate on Sprint without roaming partners regardless of ZIP code you choose.
Straight Talk Map currently shown shows only T-mobile coverage for non-android devices. AT&T and CDMA Verizon devices will have different (usually more) coverage. Android device map shows Sprint with no roaming partners.
You can get correct maps by going to Net10.com, click service and support, click coverage, and enter your ZIP Code there.
Page Plus uses Verizon without roaming partners, but still has pretty good coverage. Limited selection of CDMA phones. You can bring in non-prepaid phones from Verizon, sometimes.
$29.95 1,200 minutes, 3,000 text, and 100MB Data
$44.95 unlimited talk and text, 20 MB Data
$55 unlimited talk, text, and 300MB Data
Overage: 5 cents per minute, 5 cents per text, 10-25 cents per megabyte (depending upon plan)
There are many others, but these are the top ones for inexpensive unlimited plans...