How to make a best diversified stock portfolio?

You would choose a stock that is inversely correlated or at least not positively correlated. Use a correlation coefficient which you can calculate yourself or find websites which do it for you. If you want something less mathematical, choose stocks that tend to do well when your chosen stock does badly and vice versa.

Pears

CSX a railroad stock - what CSX handles is the basic commodities that are the opposite of Apple technology - low tech, and in high physical quantities. It pays a dividend too! "The company transports crushed stone, sand and gravel, metal, phosphate, fertilizer, food, consumer, agricultural, automotive, paper, and chemical products; and coal, coke, and iron ore to electricity-generating power plants, steel manufacturers, industrial plants, and deep-water port facilities. It also provides intermodal transportation services through a network of approximately 50 terminals transporting manufactured consumer goods in containers in the eastern United States, as well as performs drayage services, including pickup and delivery of intermodal shipments; and trucking dispatch services." I assume you are planning to add a third and fourth stock etc as funds are available.

You can't diversify much with two equities.

You can't be diversified with TWO stocks...