If you are interested in the absolute best Thai food ever, I would highly recommend Thai D'Lish at 2447 Hwy K in O'Fallon, MO. (It is hidden in a strip mall behind Fritz's Frozen Custard.)
I have never been a "regular" anywhere, but we actually go to this restaurant at least once a week. Not only is the food incredible, but the owner and the staff are extremely pleasant, and the environment is clean and nicely decorated, and strangely enough, the background music selection is diverse and wonderful.
Due to its non-obvious location and some problems with lack of visibility on yellow pages and the web, it just does not get the traffic it deserves. I would highly encourage you to try it.
There are 3 Thai restaurants on Hwy K -- this is by far, the best one!