Define "best".
I'm using Anveo. They're actually the provider for some of the smaller VoIP services. Very good prices, very flexible with good support for multi-line businesses, and (if you know what you're doing) straighforward to set up. OTOH if you want tech support who handholds you and an easy-to-use GUI interface which hides most of the complexity and options, look elsewhere.
https://www.anveo.com
Also, unless the VoIP phone has specific features you need (multi-line switching, display, intercom, hold music, etc), you may be better off just getting a Obihai Obi200 to convert the SIP connection to a regular phone line, and plugging in a traditional landline phone handset.