They are all difficult in their own ways. And depending on the genre the tactics change too!
Modern sims are very complicated, while WW2 sims are old school complicated with math and techniques.
Modern sims take away the math, but require the finesse of maneuvers and timing.
WW2 sims dump math on you, and require setting up a desired position to ambush from. Mostly preparation!
Which do you prefer?
I have lots of videos for the SH3-5 series if you need them. Many other members have them too!
http://www.subsim.com/radioroom/showthread.php?t=240116