I've eaten a lot of Thai food, but the best Thai food is a subjective choice. I have lots of favorites of course, most of which are simple like Som Tam, the crunchy papaya salad that's so good. Those crisp julienned strips of papaya soak up the lime spicy, garlicky juice for a delicious and healthy food.
Another simple classic is sticky rice and mango. There are a couple species of mangos grown in Thailand which are sweet and make the perfect dessert choice. Of course, some prefer durian, or another fruit, but mango is the classic. The best I ever had was at a stand next to a Hilton Hotel in Hua Hin.
But this is probably my favorite Thai food of all. It's a spicy basil stir fry with ground chicken and fried egg. Why's it so aroi? I could write a thousand blogs about this, but I'll try to sum it up in a few sentences here.
There are more than one variety of basil used in Thai cooking; one of them called Kaprow is very spicy and hearty - it stands up well to stir-fry dishes. It has an amazing taste unlike the bland Italian basil you find in supermarkets. So "pad kaprow" is usually translated as "stir fry basil" and you order it by telling the server what type of protein you want.
I order this "gai sap, kai dow" which means with ground chicken and fried egg. American restaurants tend to serve dishes with thinly sliced chicken pieces, but I prefer ground. I've asked why before too - the answer I usually get is that slicing thin, wide pieces makes it look meatier.
The egg is going to be weird for farang (westerners) too because this egg is flash fried in hot oil in a wok. The egg cooks so fast the outside is crispy/crunchy, but the yolk is still runny.
So that's my favorite Thai dish, usually called "chicken basil" or similar. You can order this in any Thai restaurant too - just ask your server about it. The good thing about Thai restaurants in the West - they always seem to be run by Thai people, so you can order the authentic stuff! In fact if you can read Thai, ask to see the Thai menu - it'll have smaller portions for cheaper prices.