My absolute favorite has to be Airline Mogul. There are lots of worlds to choose from, each world takes place at different years and starts you off with different airplanes. It's very realistic with aircraft performance, finance and variety of airports. Best of all, it's free, well, you get free tokens every 12 hours, each world costs 30 free tokens and you start with 60 tokens. You can buy tokens if you want to create your own private world, where you can choose what planes you start with, how much money you start with and what parts of the world you and the amount of people you choose that can be in the world can fly to. I really recommend trying it out.