The best toys are the ones a child returns to. That usually means open-ended: a set of blocks beats a single-function gadget, and a ball outlasts almost everything.
Start with the age range on the box, then ignore it slightly. Age ranges describe safety and dexterity, not interest. A four-year-old with a steady hand can manage a 6+ construction set with someone sitting alongside.
Finally, check what happens after the novelty wears off. If the toy has a second mode of play — building, sorting, racing, collecting — it earns its shelf space.