This is off of ign. I followed it and got a perfect rating.
The rating depends on three categories: Cleanliness, Abundant Nature and Abundant Living.
Cleanliness
less than 10 weeds
less than 10 items on the ground (not counting sea shells, fruit, mushrooms, flowers, buried fossils, buried gyroids, buried Pitfall Seeds)
no garbage (Empty Can, Boot, Old Tire, Spoiled Turnips)
no Rafflesia
Abundant Nature
Trees between 110 and 200
Flowers over 50
High Nature Points from Public Works Projects
Good: Flower Clock, Flower Bed, Flower Arch, Wisteria Pergola, Straw Fence, topiaries, Solar Panels, Wind Power Generator, Windmill
Bad: Garbage Can, Jumbo Monitor, Oil Excavator, Tire Play Equipment, Picnic Sheet, Traffic Light, illumination items, Tower
Abundant Living
High Living Points from Public Works Projects
Best: Campground
Good: bridges, benches, outside lights, Garden Chair, Hot Spring, Fire Hydrant, Stadium Lights, Solar Panels, Lighthouse
OK: almost everything else including Water Well, Fountain, clocks, Signboard, Face Board, signs, art, monuments, etc.
Bad: Garbage Can