ATK + SPEED
Go to Lavender Town inside the tower, go to the floor above the nurse i think its maybe F4, kill pattrat, rattata, spinark, only 1 point each so bit slow but almost 100% chance to meet one of them, and nurse 5 sec away so pretty nice. Its also gengars and ghastly so for sp.atk it might work on another floor, havent tried.