The best coloring page is not the hardest one. It is the one a child wants to begin and can decide how long to stay with.

For a first start, clear outlines and large areas—a fish, balloon or simple animal—make it easy to see where color can go and to notice a change quickly.
Small scenes offer more to discover as experience grows; detailed mandalas or nature patterns suit children who enjoy staying with a task longer. Older children may still prefer simple pages.
Before printing, ask whether the main motif is immediately clear and whether there are areas the child will enjoy filling. Those answers matter more than a strict age label.