The first Opeth song I ever heard was The Drapery Falls. I was reading reviews on Amazon and stumbled upon Blackwater Park. Some guy wrote a review that focused entirely on TDF without mentioning the rest of the album. I thought this was funny, but it intrigued me enough to make me listen to the song. I was floored. At the time, it was the most amazing piece of music I'd ever heard. (I was a hardcore metalhead at the time; all I listened to was Pantera, Slayer, Megadeth, etc). The Drapery Falls is now my 3rd favorite Opeth song, with only Deliverance and Ghost of Perdition beating it.
So from there, I got Blackwater Park. I credit that album for getting me out of my "metal-only" rut. It opened up so many doors for me to discover new music I would have previously ignored. Prog mostly, but it taught me not to pigeonhole my interests into one genre ever again. The second album I got was Ghost Reveries. Oh boy. I'm not even going to try to describe how much I love that album. It's my favorite album ever. I've listened to it more than anything else, and I never get tired of it. I go back to it all the time and I'm always stunned at how perfect it is. I could listen to it on repeat for the rest of my life. By extension, Ghost of Perdition is my all-time favorite song from any band.
After those two albums, I realized how special this band was, and I got everything they've released. (Minus the Lamentations DVD, I still need to get that...) The amount of quality music this band has put out is unsurpassed. I may not be big on Orchid, but I adore everything else, including Watershed. I sometimes playfully knock on that album, but it's only because it had the unfortunate task of following Ghost Reveries. It's still a nearly perfect piece of work.
Pinpointing what makes this band so special for me is difficult. It's one of those things you feel that's impossible to put into words. If I have to try, I'll say that the atmosphere the music creates, and the emotions it makes me feel, is what keeps drawing me back to Opeth. The greatest thing about this band is their marriage of beauty with ugliness, gentleness with brutality, and good with evil. They make me feel as if it's ok to feel extreme emotions of any kind, as long as there's a healthy balance. They cover the whole spectrum.
Basically...their music feels complete, and it makes me feel complete. When I'm listening to Opeth I can feel powerful enough to climb a mountain one second, evil enough to slaughter a puppy the next, only to be brought back in by the feeling that I'm at peace with myself, and that the world isn't such a bad place after all. Listening to Opeth is an emotional roller-coaster ride, but an immensely satisfying one. I never get as involved in the listening process as I do with this band. It's so rewarding to just kick back, relax, and let Opeth's music take me places and teach me about myself.
So yeah...I think that covers it. Opeth is awesome.