Ditto, jar it comes in on longer trips, with the caveat that I like to buy these at places like gas station mini-marts, as they sometimes carry smaller sized jars, i.e., a 12 oz rather than an 18 oz jar. I've noticed more often recently that even the gas stations are carrying the larger sizes now, so if you do get a smaller jar, consider cleaning it out and reusing it.
Related factors are how much peanut butter you eat, whether you're sharing food between 2 or more people, and how many days of food you're packing.
I miss the old 0.9 oz singles and the 9 oz squeeze tubes, I think these are hard if not impossible to find now (?), and the little single-use units for sale in stores now aren't very good for packing and generally the wrong size for me.
An alternative is to look for alternatives to peanut butter. Refried bean powder works well for me; while it's more on-trail fiddle factor, it's lighter, tasty, and provides the protein I'm looking for.