https://www.milsurps.com/images/impo...3c00917d-1.jpg
Whatever you can get in the bottle!
ETA:
Having re-read your post, the pepper vinegar isn't added during canning. It's added after cooking! Use whatever peppers you like, mild to wild. When the juice is gone out of the jar, add some vinegar again to whatever peppers are left. Let sit as long as possible and it ought to be as good or better the next time or three! A few jars in rotation works well. The flavor's good even after the peppers get a bit "less than fresh" looking.