Either of the Yak-9s.
The Yak-9U had the best combination of speed/maneuvering in the game (IMO of course), although the weak guns (1 ShVAK and 2x .50s) and limited ammo are a turn-off for most players.
The Yak-9T is just funny. It handles well at higher speeds, but doesn't have enough horsepower to keep it up (so it's a bit of a bobsled, great downhill, but you have to push it back up) The funny part is the 1 ping kills of pretty much any fighter in the game (due to the 37mm elephant gun). It only has 32 rounds of ammo, but remember, it only takes one round to get a kill lol.
-Sik