Who do people think are the worst match ups for Nole (in theory at least - whether or not it translates to results)?
1. Djokovic doesn't like sliced backhands
2. it's possible to grind out Djokovic, he can be bored (or sometimes tired) in a long rally and make an easy error
3. it's good to have a good forehand (if you're a righty) on high balls because he likes that spinned cross-court forehand, you also have to be able to play well on deep balls, being able to half-volley is good
That said, I think not surprisingly Murray comes here, as a player who neutralizes him very well, Dolgopolov as well his sliced backhands and accelerations are good against Djokovic. Tomic as well could work imo if he improves his movement.
That's the first three names I think about.
Federer on a lesser extent but he needs to avoid errors and shorten the points because his backhand is not good enough against Djokovic.