S-1 is what real Muay Thai looks like. S-1 had plans at one point to come to the U.S., but as you can see from this video, some of the more brutal strikes (spiking elbows to the back of the head) will probably have to be nixed. That would basically turn S-1 into modified K-1, so I'd rather they just stay local.
Thai boxing is unbelievably huge in Thailand and S-1 is the largest, but also the most authentic place to see what actual Muay Thai actually looks like. There are over 100,000 professional fighters competing in Thailand, and judging from the highlights in this video, everyone of them is a beast.