At the moment, my favorites for Smartphones are:

Verizon Network: Verizon's prepaid (Allset), Pagepluscellular.com

AT&T Network: Cricket (runs on AT&T network), or StraightTalk-- See below how to get AT&T network version.

One of the good options for prepaid service is Tracfone or Straight Talk. Finding these options is easy. These are often found in variety stores (i.e. Walmart) and in some smaller convenience stores like 711. The feature-phones (cheaper varieties, non-smartphones), tend to be in plastic and cardboard hung on hooks for grab-and-go convenience.

These folks resell access to the major networks (i.e. AT&T, Verizon, T-Mobile, Sprint). In any event, one gets better geographic covearage on either AT&T or Verizon. They simply have larger native networks. Although Tracfone and Straight Talk do not advertise which carrier the phone they sell uses, it is possible to determine which one by looking carefully at the phone's packaging. For now, I always get the AT&T phone. This is because Verizon doesn't permit resellers to access its high-speed data network. AT&T will permit a reseller to access faster speed data (with similar if not quite as complete coverage as Verizon).

"Most Tracfone, NET10 and Straight Talk phones also have a network code on the box. It's usually in the lower left corner on the front of the box. These codes are: CDMA-S = Sprint, CDMA-V = Verizon, GSM-A = AT&T,  GSM-T = T-Mobile.

TracFone Market codes. Decode them as follows:
GSM4 = AT&T
GSM4SPR = AT&T or Sprint (Net10/Straight Talk Only)
GSM5 = T-Mobile
GSM5AT = T-Mobile or AT&T
GSM5SPR = T-Mobile or Sprint (Net10/Straight Talk Only)
CO = Verizon
COGSM4 = Verizon or AT&T
COGSM5 = Verizon or T-Mobile
COSPR = Verizon or Sprint (Net10/Straight Talk Only)"

See http://www.prepaidphonenews.com/2011/12/how-to-get-tracfone-net10-or-straight.html for more detail.