There are slight regional differences in Muay Thai the sport, mostly based on coaches preferences and emphasis.
When we start talking about the barenuckle styles then they are are different styles depending on location. Currently I don't think anyone on this forum has experience enough in the barenuckle styles of Thailand to make any kind of educated comparison.
There are also combative forms of Krabi Krabong, but this is basically a different art.
Basically you have a single style based off of military effectiveness and then over years it branches off in different directions to fulfill different purposes...now you have regional styles, you have Krabi Krabong, you have Muay Thai, etc.
If you are looking for the "best" Thai style, I'd suggest learning Thai, moving to Thailand and start training. Look for the style that is least diluted from the original battlefield style.
Also, don't hold your breath, because its going to take a while.
