What smartphone, service, hotspot, booster do you use?

I'd say the answer depends a lot on your needs.

If you don't need much in the way of phone or internet service on the road--say just calls to make and confirm reservations, checking the weather forecast once in awhile, having something to use for emergencies--it's hard to beat a Tracfone on price. Their lowest cost options end up being a little under $10 per month, with pretty limited data and talk time and texts that do roll over indefinitely as long as service is kept active.