I'd have to say as far a s "The Best" goes, there isn't one. Some guys are supernatural in one ride and average in others, some are above average in all but still not the best in any... I dunno, sounds weird, but thats how I feel. Plus you throw in the occasional run of good/bad luck, and it can totally screw anyone up.
Having said that, some I cringe to fight against...
Yucca in a P-47
Killnu in a P-38
Jaekart in a Hurricane (either one)
most of the 4thFG guys in a Pony
Doom in a 190 (easy to outthink sometimes, but he'll usually make you work for it)
N7 in any LW fighter
Pooface in a Spit (not unbeatable, but again will make you work for it)
Storch in a 110
But I have to say I still haven't seen anyone who scares me as much as some of the guys I used to fly WB with- I remember when DADDY ran up about 500 consecutive kills in a P-51D, Dracer taking out about a half dozen A6M3s in a P-40B, Dawger taking on our whole squad (8 of us in 109G6s) in a P-38J and killing or crippling every one of us and still making it home, SiloFT dogfighting Corsairs with a Val and winning...
Ah, memories...