These are road marathons. Trail marathons, especially single track technical stuff, are different things entirely.
1. Hogeye in Arkansas. Hot and hilly.
2. Heart of America in Missouri. Also hot and hilly.
3. Mt Desert Island in Maine. Hilly and generally dismal (wind rain) conditions.
4. Grandfather Mountain in North Carolina. Very hilly and stressful course.
5. Big Sur in California. Hilly. I really don't think this race is as hard as some people describe. There's a trick to it. Because of the windy road next to the ocean, you can see the long uphills for several miles in front of you. This is difficult psychologically, even though these hills aren't really any worse than hills in other races.