It's probably impossible to give a reply here that doesn't have a bunch of exceptions, but I think what the last 20 years or so have seen is that if you build a pipe with special attention paid to the construction of the airway, making it smooth inside with very little in the way of open spaces (plenum spaces) or transitional points, you get a good smoke.
What are we trying to do except carry a fairly hot stream of molecules to the smoker's tongue? The "flavor" in smoke is big molecules, esters, oils. If these don't make it to the user, the smoke is.... steam. So you want to build a pipe that will not cause condensation in the smoke stream, no bumps, no lumps, no changes in the tube size (which would make for non-laminar flow and/or a pressure change). Keep all that tasty stuff in the smoke stream!
I think the American makers of the last 20 years really zoned in on this, and at this point, almost every serious pipe maker takes care to build a very decent airway.
Pipes that don't work well usually have some physical flaw in their build. I have owned probably 100 pipes, and the good ones are identifiable by build. Brand is basically nothing, construction is everything (that said of course, certain brands do a more uniformally good job in construction - Castellos for example are generally speaking a very nicely put together pipe, and the result is that lots of people think they smoke really well).
Briar is ... some factor. For sure not every piece of briar smokes or tastes the same, but for every person that like a super light piece of briar, you'll find another person that prefers more dense stuff. For everyone who swears by Greek briar, you'll find someone who prefers Italian. It's a very small difference, I think I could tell you on the first smoke or two if a block was Algerian vs Italian, but after 10 smokes? I don't think anyone can tell from the smoke.
I sought magic briar for years, bought and tried everything under the sun. At this point, I try to buy clean briar that is well cut, but I buy from multiple sources because they have all provided me with excellent looking and smoking pipes.
It's very, very difficult to sort this stuff out retro-actively - you'll find a guy with a 50 year old pipe that smokes great and it's stamped "Algerian Briar" so naturally he thinks that's the best - could be, or it could be that the airway in the pipe is super smooth after 50 years of pipe cleaners going in there, and the physics of the pipe are just really good.
My pipe making journey kind of began when I did get a really good pipe, a pipe that stood out as an excellent smoker - non gurgler, tasted good, seemed to stay lit effortlessly, seemed to smoke every kind of leaf well. What the hell could be going on? Once I got some of these ideas of constant volume airways etc under my belt, I started looking really hard at how pipes were built, and time and time again I found that they are built very sloppily, with mis-aligned drillings, rough finishes inside stems, poorly cut slots etc. And those pipes always smoked crappy.
So I will submit that a pipe is like... 95% physics, a fluid-dynamics equation, and 5% magic where things just.... work out "extra" right sometimes.