For me it was Advent.
This was the first Opeth song that I heard. Advent is the one that did it for me. After listening to this song and the whole Morningrise album for that matter, I went out and bought the CD, shortly after I got MAYH and Orchid, which was like four years ago. I now have all the albums, except for Deliverance,( I have only heard two songs off that disc: Wreath and Deliverance)
When I first heard Advent it was just amazing though, it was just really refreshing compared to the stuff that I was hearing in that time period(1998) The guitar style, the acoustic interludes, vocals, the bass( mainly due to the fretless sound) the structures, sudden breaks, and just everthing in general was quite a piece of work to say the least, same goes with the rest of the disc.
Morningrise is a great record, I just never find myself going back to it much, just on occasion and I mean OCCASION, same with Orchid, there both amazing albums, they just don't have much replay value for me like My Arms Your Hearse, Still Life, and Blackwater Park(I'm excluding Deliverance) These I still listen to quite a bit especially MAYH and Still Life.
I now await Deliverance with much anticipation.