I've been pretty happy with Visible which is a Verizon owned MVNO (sort of, not sure if it's a true MVNO if Verizon owns them). Everything is done via an app,

I've been pretty happy with Visible which is a Verizon owned MVNO (sort of, not sure if it's a true MVNO if Verizon owns them). Everything is done via an app, you get the Verizon network with unlimited LTE (though you do get de-prioritized during congestion, but I've found it to be fine in the bay area). It's $40/month which includes all local fees/taxes and it's no contract. Only downside is no cellular support for the Apple Watch.