David you took on a big topic when you asked who it the GOAT in US karting. Different eras, racing formats and areas of the country are going to make this a tough one to answer. I never saw Kathy Hartman race, but I have no doubt she was one of the best by what I have been told and what I have read. I have seen Lynn Haddock race and I have no reluctance in giving him consideration. Lake Speed winning the World Championships make him a contender as well.
My time in karting was primarily consisted of street racing in the 80’s and 90’s. During that time there were none better than Mike Birdsell, Scotty Evans and Ron Jones in the streets of the Midwest. All these guys had success on the dirt and sprint racing as well. B.J Carter and Jodi Krug are two more Midwest dirt racers that were as good as they come for 20+ years. Krug still suits up once in a while and runs at the front.
Lastly, I never saw him race, but I have it on good authority that Barnesville legend Dicky Wilson was one of the best in his day.
It is obvious our sport has had many talented drivers, but if push came to shove and I had to cast my vote for just one–it would be Lynn Haddock. He has raced and won in virtually every form of karting known and he did this for decades.