Here's an idea: How about zero days?
I admittedly don't get how this is even a thing, having bought unlocked phones for prepaid service going on 14 years now. Wait for a sale on a phone, get a high-end device for like $800 (financing always available), and pay $200 once a year for service.
It's appalling to me that people think more than $17/month for cell service is reasonable.