Probably Net10, they are bigtime partners with Walmart (but could also be Boost Mobile or a couple others with similar plans).
I bought an unlocked MotoG and with Net10 I pay either $40 for 3GB or $50 for 5GB depending on my expected usage for the month (you can get bigger plans if you want at $10-for-2GB increments). If you go over you are not cut off, just restricted to 2G service (latency is horrible though). Tethering is technically against the rules but they don't really care, I've never gotten in trouble. You either get carried on T-Mobile or ATT.
I do ATT and it works fine - I am up north right now in Bumfuck MI (a cabin in the middle of a National Forest) and I have 4/5 bars of 4G signal. I think coverage worries are a thing of the past unless you are using one of those carriers that is specifically city-only. I get around and it's just never been a problem for me.
I buy refills through Callingmart. They don't charge all the extra fees that Net10 does directly (typically an extra $4/mo or so), they do 1% loyalty cashback for your next order, and they constantly run 3% off coupons (check retailmenot.com). So for a $50 refill that costs $54 though Net10's store I am at $46.50 out the door. They have refills for tons of carriers, worth checking out if you're monthly.
I said it below and got down voted but if you aren't unlocked and monthly you are getting screwed hard. My phone is rooted and has an unlocked bootloader, so I can run CyanogenMod (i.e. I can get security updates instead of being tied to the carrier's firmware), use the radio, tether, or anything else I want. I have absolutely zero ties binding me to my carrier - if next month some other carrier offers a better deal (or my current one tries to screw me) I just buy a $2 SIM-kit, swap and activate, and point my Google Voice number at the new SIM. Takes five minutes of work and nobody else even needs to know I changed carriers. The only downside is that you need to pay for a phone upfront instead of on a phone plan since nobody will finance an unlocked phone (hello startup idea).