Which carrier has the cheapest data plans and good data reception for an iPhone?

AT&T, Verizon, Sprint, or T-Mobile.

As with many things regarding your cell phone, it depends on where you live. Some areas, such as surrounding large cities, have incredible and reliable reception. However, you can go in to those cities and find some difficulties due to obstruction of signals from the tall buildings. Personally I have made choices based on word of mouth.

Verizon hands down has the best signal, but can get pricy. Sprint is improving their signal, as they are about to launch their 4G LTE in the upcoming months, and they also tend to have consistent billing as they offer unlimited everything for $80, and up until the 11/27 you can also get the iPhone 5S for only 99.99 by switching your number over to Sprint. Tmobile and Sprint go head on on being the cheapest but Sprint is a safer choice.