Atomic force microscopy is a versatile tool for ultrasensitive measurement. We explore the fundamental and practical limits to force sensitivity that are imposed by optical and electrical deflection sensing techniques, and present a method for designing cantilevers to optimize force sensitivity. We calculate the optimized force sensitivity to be 1.2x10(-18) N/root Hz at a temperature of 4 K, using a gallium arsenide piezoresistive cantilever with dimensions w=0.1 mu m, t = 0.1 mu m, and L = 81 mu m, and mechanical quality factor Q = 10(4). The optimized force sensitivities obtained using silicon piezoresistive and optical detection are not far behind.