Whats the best service provider?
I think answering "the best" provider is too broad a question to accurately answer. People will give you their opinion and experience. GrapheneOS has very good support for, I think, most all APN settings. Keep in mind that in the US, there are only 4 national networks - ATT, Verizon, T-Mobile, and Dish (Dish are new and have a very small network). Everything else is MVNO's who use the national providers' networks. I'm not sure this list is up to date, but it gives you an idea that there are A LOT of MVNOs https://en.wikipedia.org/wiki/List_of_United_States_mobile_virtual_network_operators.
What are the best providers I can get anonymously in USA?
By "anonymous" I assume you mean walking into a store and paying cash for a sim card and refill card. Off the top of my head you can do that at Target and Walmart, so you can look at which plans/providers/MVNOs they offer.
I hope this answers your question, or at least points you in the right direction.