I myself haven't experimented much but from the countries I have played in......1) USA....most of the times you will get many domestic routes with high passenger numbers and returns and no competiton.....don't be scred by the high number of airlines you se....their are only maybe a max 7-8 airlines active at any given point and most of them are busy expanding internationally or even if they do have a wide domestic network....you will still get a lot of free demand for yourself
2) India....Man if you do complete justice with its domestic network....you have a pretty good daily operating profit and atleast 3 out 5 times....you don't have a good enough competition there
3) Brazil....Same as India
4) Saudi Arabia
5) China...Same as USA
6) Indonesia and Vietnam.....here you might get no competition at all