Keith Law offered up his mid-season Top 50 list (Insider-only) today. Notably, Mark Appel is included. Since it's behind a paywall, we're not going to pull any write-ups from it, but here are the Astros' rankings:
#7: Carlos Correa
#9: Mark Appel
#29: George Springer
#33: Jon Singleton
#46: Mike Foltynewicz