First, understand that you don't have to replace your existing handsets to reduce your phone bill, despite the fact that most people are going to try and suggest options that will require you to do so anyway to "save". Nobody factors the added handset cost into their "savings" where they have to buy in with a $200+ phone to switch providers, and often slightly more expensive providers that let you keep your existing handsets will prove out to be cheaper long term if you keep the hardware you already have.
No such thing as an overall "best" though, only the most serviceable option for the individual. Anyone who claims there's a magical one-size carrier that works for everyone is either naive or lying. People have to choose a solution that works best for them.
Enter the guide, which helps you cut through the malarky, figure out what *you* need, and find a carrier from a smaller curated list that works with your existing equipment at a decent price who treats you well and actually values you as a customer. Remember, don't pay for "unlimited" plans unless they make financial sense... otherwise only pay for what you need. You might save just as much or even more that way without compromising service or getting into a raw deal.
Give the guide a read, understand what you need and the price you want to spend, and pick a Verizon or AT&T MVNO (depending on the coverage you need) that allows you to keep your existing handsets to keep electronic waste and needless consumption reduced. Selectel is about the only one on the short list for decent Verizon MVNOs, but unless you need Verizon specifically for the coverage, AT&T MVNOs are a solid second choice under most circumstances. Of those, Airvoice, Puretalk USA, and Consumer Cellular are the most solid.
If you have any questions, I'll be happy to help you out.