What's the best treatment for sacroiliac pain?

The best treatment. Is the one that fixes the underlying cause. In the case of sacroiliac joint dysfunction the underlying cause is pelvic imbalance due to either unequal spasm of psoas muscle, short leg syndrome, bad chair at work or car seat. Arthritis in this joint can cause pain, but usually not in your age group. See a good osteopathic physician or a physical therapist.
Generally an NSAID. Usually an anti inflammatory pill but sometimes a cortisone injection into the si joint.