ive always thought about it, and everytime i do, i end up thinking its four different people then the last time....

for sure i think one is jim.
for some reason, i dont see dwight liking creed all that much...
maybe ryan...hogging all michael's attention and all...
lastly....kevin? i dunno, i cant really see them getting along.

anyhow.....who knows what goes on in dwights mind???

Who I think it is:
Jim for the pranks.
Ryan because Michael seems to like him more than Dwight.
Toby because Dwight strives to be like Michael.
Kelly because, really, who does like her?

Who I think it probably isn't:
Michael obviously.
Pam because of when he made her Secret Assistant to the Regional Manager, he seemed to like her.
Angela obviously.
Oscar because he knows that Oscar owes him "one big favor redeemable at any time" by him, so he treats Oscar nicely.
And basically Stanley, Kevin, Phyllis, Creed, and Meredith have done nothing to cross him yet.

its probably
Stanley - he doesnt respect dwight at all
Ryan - for inadvertantly hogging Michael's attention
Meredith - he says that she wouldnt be missed
and of course
Jim - because he's Jim

It's definately not Angela, for obvious reasons, and it can't be Kelly because he had a crush on her.

My guesses would be...

Jim: Obviously

Meredith: I base this on his attempt to get her fired in a "The Coup" deleted scene

Ryan: Due to Michael's love for him, I could see Dwight harbouring a dislike for him.

And I have no idea who the forth could be. Maybe Pam (Jim's usual accomplice)or Toby?

(This list would have been much easier to make if only Andy had been working with him back then)