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 or a vampire ..etc
I think that the Winchester boys are the easiest to hunt cause they are so hot. Oh you 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!!