Beef, chicken, pork or fish bones with some meat on them
Salt to taste, but you can add it later (I like to add a dash of fish sauce before drinking it)
Apple cider vinegar (lots of recipes call for this, not sure why...perhaps the vinegar extracts more nutrients?)
Organ meat (liver is excellent)
Aromatic herbs (Bay leaves, star anise, peppercorns, YMMV)
Other saved meat juices (i.e. from sous vide anything...I pour the juices into a glass jar and keep it in the fridge all week long...just dump this in with the rest of the ingredients)
(optional for beef/pork bones) Preheat oven to 200 degrees Celsius, spread bones out on a tray and roast for 45 minutes or so til they get some nice crusties for flavor and darker color. Might work good too for chicken/fish but perhaps less time needed.
Add ingredients to pressure cooker, fill to cover bones (no higher than Max line), pressure cook on high for 40 min - 1.5 hours (depends on the type of bones, but doesn't really hurt to go long). Quick or natural release depending on whether you want to leave it longer.
Sieve the solid material out. I have a large steel bowl on top of which I place a colander. I ladle the large bones into this and drain, then put the material into a large ziploc back to freeze before throwing it out with the burnable garbage. Then pour the remaining broth through the colander. I don't worry about getting all the particles out of the broth myself.
Some just put the bowl of broth into the fridge, but the issue is cooling the broth before it develops dangerous bacteria, and large amounts of broth in a fridge/freezer will not cool fast enough. Some recommend an ice bath for the bowl with the broth, but I don't have a pluggable sink and don't like wasting so much ice and water. Instead I have 2 cup size ziploc plastic containers. I ladle 2 cups of broth into each (perfect amount to split with my wife) and put these into the freezer immediately. Smaller amounts of broth cool faster and safer, and all we need to do is put a container into the fridge each day to defrost for tomorrow.
Speaking of which, another benefit of this system is the fat coagulates in a nice disk on the top of the container. If you defrost the broth in the fridge, then you can easily remove the fat from the top and use it for other things (like mixing in with scrambled eggs) to get more fat calories. If you just microwave the frozen container, the fat will liquefy and it's a pain to remove without a fat separator tool, which is also a pain to use and plain old messy.
Beef bones: Beef ribs are excellent as they have lots of bone and meat on them. Bone broth should include some meat as a lot of the extracted vitamins come from the meat as well. Soup bones are ok, but usually have more grit and less collagen to extract. I'm going to try a bag of some serious beef bones I bought at the grocery store last week on Saturday. Looking forward to it.
Chicken bones: We often buy a roasted chicken from the grocery store and eat what we want off of it, saving the bones in a bag. Then we throw the whole carcass along with left over bones into the pot to make chicken broth. So good.
Fish bones: Planning on doing this soon and really looking forward to it. Key here is to have a nearby supply of fresh fish bones, heads and tail being particularly good. Some meat on is good too to get the fish oils and such. Luckily here in Japan we stopped by our local fishmonger and found several cheap packages of fish heads/tails/etc... that will be perfect.
Pork bones: I'm guessing same thing here as for beef, but honestly haven't tried to make a pork specific bone broth yet. One of the main types of ramen broth here is tonkotsu, made from pork bones. But they are slow cooked all day til the bone particles come apart and create a milky stew which is delicious. Takes a lot of time and not sure how well you can mimic this in a pressure cooker.