I buy pipes based on what tobacco I am going to smoke. Since I smoke flakes and plugs 99% of the time, I buy pipes that I know will make the tobacco taste great because of the technical criteria I stick to on my pipes. Almost all of my pipes are group 4-5 with a few 6's mixed in.
For example a basic group 5 pipe has dimensions like these. Bowl height 2.0", inside bowl depth 1.5", inside bowl width .75-..90. I like a pipe that is 5.0=6.5 in length and weights of around 40-60 grams. I fold and stuff all my flakes and these dimensions work great almost all the time. A few of my flakes they are wrap arounds and those can be a little tricky. Plugs I cube cut and pack them so there is some resistance and a bit of moisture. All of these criteria took years to figure out and yes I butchered plenty of bowls. I now never light a pipe until I check the draw. If it is too tight, I run a pipe cleaner through it and that usually opens it up just fine. If it is too loose I just tamp down some. Yes there have been times when it is way to tight and I have to start over again, but that is a rare occurrence .
All of my pipes except 2 are all classic shapes like Billiards, Apples, Dublins, Brandy's, Rhodesians, Bulldogs, Lovats and I think that is it. My collection could be called boring, but these pipes smoke my flakes great and I like the way they look as well. There are probably more shapes I dislike than there are that I do like. Here are a few of classic shapes I really like.
Here are is a Jack Howell Billiard
Here is a Rad Davis Rhodesian.
Here is a Larrysson Dublin.