Hands-down, the assist meister are the 109s, w/o MK108, w/o gondies. It's a great useful plane for point defence, immediate air coverage and when there are multiple 109s around one really notices how much defending becomes easier.
But the damn frickin' armament usually means that the 109s will work their prettythang off to bring higher planes down to deck levels, where either the La-7s or Spits are waiting to finish the enemy off, depending on whether the target decides to turn or to run - whichever case, the 109 rarely gets to kill anything.
I think the highest assists I got was about 9 in a G-10.