What is the best plant or shrub that grow fast and cover big rocky hill?

What part of the country you live in will make a difference. If dry area, a succulent is best. If a humid area ivy is bad since mosquitoes love to breed in the water that ivy holds in the leaves. Lamb's Ear spreads like crazy, but not sure if it does well in the north. DayLillies spread and are pretty. Ask at the local garden center what is best for your zone.