What would be a good substitute for Aleppo pepper?

used in a crème fraiche glaze on chicken

A Spanish pimenton de la vera would work or piment d'esplette. Supermarket paprika would be a very bad idea; save that for deviled eggs. But as we don't know what dish this is going into it's hard to be more specific.

Since it's generally milder than chile pepper or cayenne pepper, you would need to reduce the amount from the amount of Aleppo called for. You will also lose the smokiness.

chile pepper would work as a substitute

http://food52.com/hotline/5720-what-is-aleppo-pepper-and-what-can-i-use-to-sub-for-it