As stated, 'fear' isn't the right word.... It's a game and it's fun, so fear doesn't come into it.
Having said that, I do agree with the OP, the Ki-84 is the greatest challenge for me, it's one of the few fighters that can take the fight to the 109 in the verticle. It really comes down to a pilot versus pilot fight and if I want to get out of a tight 1v1 with a Ki-84 I know that I have to make all the right moves... no mistakes and no missed shots.
Typically, Ki-84 pilots have a little more skill than your average dork in a Spit16 or La7 too, so I always go into the fight assuming that the guy has a decent idea about what he is doing.