You'll get responses for a wide variety of airplanes, mainly due to the different type of airlines you can run in AWS.
Usually when a game-world is just starting, if you want to run a big airline the best way is to get a large plane like an A300 or a B757 in order to get the most revenue per flight.
But given that the game world is quite in a cruise mode but still lots of competition (and as bigger planes struggle against smaller planes), any mainline airline starting-up would be better off getting some 150-seaters (like the 737, A320 or MD90), but always taking consideration of the availability of these aircrafts in the used market and the length of the production queue.
If you're planning on going regional, there're lots of types like the ATRs, the Q400 or the Saab 2000 which are great for that kind of flying.
Avoid regional jets.
If you plan on going international from day 1, it's much more complicated but a good 767-300/400 or the odd A330 you can find in the market would be solid takes (again, take in consideration the possibility of getting more of them from the used market uneventfully - You won't want to add fleets to the soup, otherwise youŽll end up in the red pretty soon).