Tom yam gong; The most famous soup / dish of the Thai cuisine! It is a herbal soup, with mushroom, shrimps and Thai spices. Some people prepare it with rich coconut cream while some prepare the soup without this so that the soup stays clear. Both styles are very Aroi maak maak (very delicious in Thai) :p
Beware that is often very spicy.
Tong ga gai; Personal best choice of Maurice. This coconut soup with chicken, mushroom and Thai spices is a great dish together with plain rice and any other dish.
Keng que wan; famous Thai green curry. Warning: very spicy! This sauce is prepared with eggplants, mushrooms, peppers and meat of choice.
Papaya salad; somdam; This spicy salad originates from the North Eastern part of Thailand. With raw papaya, Thai onion, peanuts, dry shrimps and special spices which are smash together. It is also available with soft raw crab.
Naam tok moo; Also from the NE Thailand area (called Isaan). Sliced pork meat or meat of choice with onion and spices.
Interested in more? read up at: wikipedia