Most som tam in tourist areas is basically fruit salad. Somtum der does the real isaan version that will destroy you if you're not prepared. Their som tam poo pla ra (with fermented fish sauce) is weapons-grade spicy

Start with som tam thai if you're scared, but the real deal is som tam lao with proper chilies - only order this if you've tested your heat tolerance elsewhere first. They have multiple locations but the original near siam square is best. Costs ฿120-180 depending on version. Order sticky rice or you'll die