http://pokemondb.net/move/nature-power
Building or paved/plain terrain: Tri Attack
Sand and Wi-Fi/Linked Battles (Also PO): Earthquake
Cave or rock: Rock Slide
Grass: Seed Bomb
Water: Hydro Pump
Puddles: Mud Bomb
Snow/ice: Ice Beam
Weather has no impact on Nature Power besides making Hydro Pump 1.5x more powerful.