As far as career goes, I think BWP was their "breakthrough" album.
I agree with
BWP, as Bleak really seemed to reach out to new fans. Some may say that song is over rated, but it's really an amazing song!
I would also say that
GR was a HUGE move for Opeth, career-wise. While
BWP reached out to new fans,
GR seems to have solidified itself in the huge metal world, beyond the metal "underground".
MAYH is an amazing album, but still considered (to me, anyway) underground, and
SL seemed to pave the way toward where they were heading with BWP, in terms of fan-base.
WS could be considered another "career" move, for the direction in which it has taken the band. Many more clean vox and mature (?) songs. Although, these types of constant changes are somewhat expected from a progressive metal/rock band. It definitely excites me for their next work of art, hopefully another change to continue in the band's progression.
My opinion, of course...