What is the best PBX provider?

I'm curious to know, when it comes to selecting a PBX provider, what criteria should one consider in order to find the best one? Is it solely based on cost, or are there other factors such as reliability, scalability, and ease of use that are just as important? Furthermore, are there any specific providers that stand out in terms of their offerings and customer service? I'm eager to hear your thoughts on this matter and any recommendations you may have.