Perfection

Reach the summit - which only opens at 100% on the perfection tracker (check it in Qi's Walnut Room on Ginger Island). The tracker wants, in rough order of pain:

• Ship every item (Full Shipment)
• Craft every recipe (Craft Master) and cook every recipe (Gourmet Chef)
• Catch every fish (Master Angler) and find every stardrop (Mystery Of The Stardrops)
• All Monster Eradication goals (Protector Of The Valley)
• Great Friends - max hearts with every villager (beyond Popular)
• Farmer level 25 (all skills capped, Master Of The Five Ways)
• All 130 golden walnuts found
• The four obelisks and the Gold Clock built - roughly 3.5 million gold of construction

The walnuts and the Gold Clock money are the true long poles, which is why Legend tends to arrive en route. When the tracker hits 100%, climb the path north of the railroad summit and enjoy the (perfect) ending. This is the whole game, distilled - expect year 4-5 at a natural pace.