I frequent Khao San Road, and Pai is essentially the same restaurant (although no longer connected I am told) with the bonus of accepting reservations :) it is also much better for groups due to it's size.
My go-to-dish is the Pad Gra Prao, minced meat perfectly seasoned on a bed of rice with a fried egg on top and spicy fish sauce on the side. You can customize all dishes for the level of spice.
If it's your first time, you must try the Khao Soi; with chicken if they give you the option. Not sure why but it's noticeably better than the brisket. I would highly recommend sharing it because it is too rich to eat all of it by yourself.