There isn't a "right answer" to this because there isn't agreement about what "good smoker" is. There is no single set of variables that every user agrees on, no single "correct" setup. That said, there are obviously pipes that are BAD smokers, pipes that gurgle, pipes that don't taste especially good in and of themselves, or in some cases pipes that seem to smoke okay but not with much flavor. And usually you can track this stuff down to some physical problem in the build. I think speaking in general, turbulence is the enemy, it causes condensation points inside the pipe, and that's what we want to avoid. We're trying to build a very smooth easy ride for the big floppy molecules the smoke is carrying - oils and esters, sugars... "flavor" molecules. And by and large, if you do that, if you build a pipe where the smoke passage is pretty linear, smooth with no big empty plenum spaces etc... you'll find that a pipe smokes great. It's not that hard to do, the average Castello misses many of these marks and STILL smokes great, for example. It's maybe more surprising how many pipe companies produce a crappy pipe when they don't have to. But of course, dialling this stuff in takes an extra 2 minutes on a pipe, and if a pipe is produced in 15 minutes in a factory, that 2 minutes is a big cost saving. So, okay.
I've bought briar from just about everyone who sells it, made and tested pipes from all of it. And at this point, barring a few very rare examples of really foul weird tasting wood, I'd say it's the least important part of how the pipe smokes. It is a part, the basic operating parameters, but only a part. The drilling, the stem, all this stuff is more important. And because of that, makers do what they do, makers choose their own limits in terms of the pipes they'll try to build, or what they find acceptable or unacceptable in that.
Is there some secret recipe? No. I mean, here's two pipe stems, which one is going to smoke nicer? The one you can get some smoke through.
If you haven't read "Your Pipe Should Have an Open Draw", an essay in "In Search of Pipe Dreams", by Rich Newcombe, that's a good place to start. He lays out measurements that he likes. Almost every pipe being made today is made just slightly smaller than those measurements, but we are talking about a mm here or there. I would say the outside size for an airway would be something like 11/64" and the smallest something like 1/8". And just about anything in between will work just fine.