Oppo F11 Pro has extremely limited custom ROM support because Oppo typically does not allow official bootloader unlocking for this model. Because of this, you won't find a dedicated "best" custom ROM (like LineageOS or Pixel Experience) on the XDA Developers forum Why Support is Limited Locked Bootloader:

Since the F11 Pro supports Project Treble, you can technically flash GSIs like LineageOS , Pixel Experience , or crDroid . These provide a "stock Android" look and can update the phone beyond its official ColorOS 11 limit.