This is, of course, only an opinion, but I think that the best part of this story is the part where Rainsford is out on the island being hunted by Zaroff and the dogs. I think that this is the best part because it is the part with the most suspense. It is also the most interesting because it is the part where Rainsford is doing everything he can to survive.
When I first read this story (I was maybe 10), I was attracted to this part because of how intelligent and cunning Rainsford is. He figures out ways to evade and actually even to harm Zaroff even though he is the prey and not the hunter. It was very attractive because it was a suspenseful part where Rainsford struggled against the odds. That is why I liked that part best.