I've been shot down by Tempests more than any other plane, I think, so I tend to give them a bit more room than other planes.
Non-perked, I probably fear the Spit 16 the most in general because I know it's a remarkable plane.
High F4U's tend to get my attention as well, but usually I can do something better than a corsair, depending on what plane I'm in.
I don't really lose any sleep over La-7's, and frankly I don't see what all the fuss about them is.
109's tend to attract really good pilots, moreso than many other planes it seems. I don't fear them, but I will be wary until the guy shows whether or not he's a hot stick. The good ones kill me pretty often in 'em.
Player-wise, AKDG is my nemesis and has shot me down repeatedly.
Storch beat me consistently in AvA, as well as Dedalos, so I prefer not to meet them in the MA if I want to live.