The question is loaded. “Is IAX2 still the best trunk type…” Was it ever? Thinking back ten years or more, I do remember people mentioning IAX2 from time to time, but never really for its own merit, only that it’s not SIP, so if you’re struggling to get SIP to work for some reason, try IAX2.
@xrobau tweeted at me earlier in the week, “IAX2 is great for places that have a restrictive firewall that insists on screwing with SIP traffic. I have a handful of customers that literally can’t use SIP without explicit mangling at both ends to avoid sip unHelpers.” OK, but if you’ve got packet inspection ruining things and you can’t avoid it, enable TLS. Problem solved.
There will be die hard IAX2 fans out there just as there are hard core Skinny fans (looking at you, @cynjut) and UNISTIM fans (no, just kidding, there aren’t any UNISTIM fans). Can’t dissuade them. But if you’re coming at it fresh, IAX2 probably isn’t the way to start.