There are many types of rice, long grain rice, short grain, glutinious, basmati, etc etc. So what is good for making onigiri ?
Japanese rice is short grain rice which is also called sushi rice. It is also called Pearl rice in Asia. What makes sushi rice great for onigiri is that it contains a lot of starch and tends to be sticky. The rice grains will stick together when they are formed in triangle. Jasmine rice is not preferred as it is much more grainy and so when the triangle rice is formed, the onigri may fall apart.
Unlike sushi, vinegar is not put into onigiri rice. Onigiri rice is simply plain short grain rice. It is sometimes flavoured with rice ball fillings or furikake seasoning which is made from sesame seeds, nori, poppy seeds, shitake powder.