Killing low ENY birds in high ENY birds 
+2
Seriously. Almost all of my very limited time flying now is spent in either the Yak-9T or Yak-7b.... sweet little birds, and awesome high eny to turn 5 or 6 D-pony and spit16 kills into a lot of perkies. Even better if I glide home no engine, oil hit and missing parts, and have to fight the plane, physics, and gravity to get home.
I get why people love their low eny birds, heck I fly the Lala or the Tempy if the odds are really bad. But the real fun I think in this game is flying the underdog, using it outside of its comfort zone, and bringing home a bag full of late-war scalps.
I also see why people get bored with this game when they never get out of that LW plane and challenge themselves.