I would suggest The Assault on Mount Mitchell in Spartanburg, South Carolina and The Cheaha Challenge/Ultra in Jacksonville, Alabama. I've done Six Gap multiple times and AOMM is harder, mostly due to about 6,500 ft. of climbing coming in the last 27 miles. It's a unique ride in that you must be bused back down the mountain to either Marion or Spartanburg.

Cheaha, especially the Ultra, is very scenic but has sadistic climbing upwards of 13,000 ft. Both events are in May. I think they are in back to back days. Cheaha is on a Sunday and AOMM is on Monday. Heck, do both. Let me know how it goes. The Ultra was brutal last year as temps approached 90 with high humidity.

Both rides are top shelf as far as organization and support. Six Gap is too. You can't go wrong with any of them.