Rod Millen was the king for a long time, he did it in 10:04 in a toyota celica in the 90's before it was paved all the way to the top. In my mind he's still the king of the hill. Nice guy too. Had dinner with him one time. He talks funny tho
Some would argue that the Unser family are the kings, and they would have strong grounds for that argument at least from a USAC point of view.
The GT3R did it in 9:46.181 (paved, 2012), Rod's nephew holds the current overall record (paved, 2012) at 9:46.164 in a
slightly modified hyundai genesis.