Yoth, you're an aeronautical engineer? You clearly have NO idea of what subjects an engineer studies, not even what an engineer is supposed to know after the degree.
Futureng, not an easy question; the term "operating cost" means everything and nothing; every company has different needs and not every airplanes fit different "routes"...moreover, in the operating costs you have to count for the initial price, but...buying an aeroplane is not like buying a car, quite often happens that what you're looking for is not available on the market.
Roughly speaking, I'd say B737 and A320. Why? Most commons types all around the world and preferred types for Low Cost and Moneymakers companies.
FB