Depends on what your needs are. Do you stay mostly in the city or spend a lot of time in rural areas? If in the city and you want to save money go with T-mobile or Sprint. Neither will have as good coverage as verizon or ATT, but if you are mostly urban / have access to wifi you probably won't notice a big drop. If you need good rural coverage go with Verizon or ATT depending on the coverage map.
The other option that might work for you is to go with something like xfinity mobile. They use their wifi network as mini-towers and verizon network as a backup.
As an FYI:
Cricket = ATT , Boost = Sprint, Virgin / Metro = Tmobile. Everyone else is an MVNO using someone else's network.
Edit to say that most of the differentiation these days isn't so much in price as it is what you can get bundled with it. Verizon has Fios, ATT has directv, TMO has netflix, Sprint has hulu / amazon prime / tidal. If you use any of those currently may be a good idea to look further into that carrier if there isn't another deciding factor