The one I play most is wwiiol. I almost solely fly in it but theres a lot more depth to it because youre flying over terrain that has infantry and armour fighting over towns. This means half your mission will be CAP and half will be ground support (CAS). I should warn you, theres a huge learning curve and it is very easy to feel noobish for weeks as you get shot down over and over again. Theres quite a few long-term players who are good at what they do and it takes a long time to get used to the strengths and weaknesses of the aircraft. If you do try, play Axis first, they have the best aircraft until the last round of R&D when the Spit IX comes out.