If you really want a challenge, there isn't a plane in the set that can't be flown successfully. IF you take the time to set it up right, and fly it to its strengths.
If schatzi can get kills in her hurri Mk 1, you can get a kill in anything!
Yes even the val. You just have to think ahead, setup a fight you can win.
Buddy of mine named Ozark used to take the val up on Pizza map. Go over a V field in the canyons & loiter. Eventually a La7 or some such would show up.
Couple of pings with the val's pop guns then he'd fly them into a wall.
BANG, you just shot down a la7 with a flippin VAL!
Yak9's are a blast, take a bit lighter hand on the stick than a spitfire.
But fun to fly and rewarding as heck. Still possibly the most underflown plane in AH.
109's well you have enough material there to keep you busy for a year.
205 is another real challenge to survive in the main. Poor visibility, but fair gas & good ammo load.