in my opinion, some of Opeths best songs are "April Ethereal", "Demon of the fall", "The night and the Silent Water", "in the midst she was standing", or "The drapery Falls".
from orchid: in mist she was standing, forest of october
from morningrise: all of them
from MAYH: april ethereal, the amen corner
from still life: the moor, godhead's lament
from blackwater park: the leper affinity, dirge for november, the funeral portrait
Umm it doesn't matter really ... just listen to some of the songs for a few weeks .. then you'll buy all the albums .. it's a simple process ... well not really for some but... just do that