Mt. Washington in NH (race only, can't be ridden other times), 4700' in 7.6 miles. This is the toughest mile per mile of any climb in the USA, maybe the world.
Mt. Evans in CO, it is about 7000' in 30? miles
Mt. Ashland in OR is about 5800' in 25 miles, although they do a Mountain bike version that cuts the distance down to about 18 miles and keeps the 5800'

I'll give Whiteface Mtn in NY an honorable mention, about 3600' in 8 miles, but it is a toll only auto road, so it makes for really nice riding and it is right off the IMLP bike course.

I don't think your Vegas climb makes the top 10, there are probably at least a half dozen true HC climbs in CO and CA each.