First one I got postive K/D ratio in: FW190A8
I learned early on that this thing, with four 20mm and a pair of 17s, is a battleship with wings. Talk about firepower! Its a bomber buster that can also take down any fighter plane in its forward view. Climb to 20k, spot a lower target, chop throttle and dive down. Open up all guns at d500, increase throttle, and climb out with a kill on your record. Its a sure way to bust up a furball. Just don't try to follow an evading target or you'll soon end up being a target yourself. Think of flying a NASCAR race car and that's about what you get in the A8.
Second one I got a positive K/D ratio in: FM-2
Its an Extra 300S with four .50cals - pull an Immelmann, then pull another one right on top of it. Its slow on cruise but can dive with good speed. It can also out-turn most fighters it comes across. Its particularly good against N1k2s and Spitfires. P-38s take a bit longer to kill because you have to saw off more pieces before they crash. If you can catch a Mustang you can kill it easy enough. I usually run out of fuel before I run out of ammo.
Current favorite plane: Yak 9U
Blends the best and worst of my first two rides. Its fast enough to catch most planes and turns well enough to stay with a target. Its downside is that you have to get in close, real close, to have any success in it but its performance is well enough that you can chase and close on just about anything you come across. It also has a small ammo load, but that's ok because you'll be shooting at targets so close that all of your ammo will hit. Don't try spraying more than d300 out. LA-7s are faster and turn better, but the Yak-9U will get you more perk points per kill. Just remember to keep an eye on that ammo counter. No WEP either, but you don't need it. Its a real pilot's plane - you can't rely on guns or speed or armour to get you out of a jam, just good pilotage.