i don't think that there is anything easy to hunt but i think that ghosts are less difficult to hunt just burn the remains for me it's easier to hunt them than to hunt a demon ,shipshifter oder a vampire ..etc
I think that the Winchester boys are the easiest to hunt cause they are so hot. Oh Du meant monsters?? Oh Werewolfs because Dean and Sam took one down with their dad when they were just little. But let's face it anything in a Winchester's way is going down.... Hard!!