"Blackwater Park" is really good, and is probably what most people would say to start with, but I could never get super into that album for whatever reason. The one that got me really into Opeth, and was my favorite for the longest time, was "Still Life." But for a while now, "My Arms, Your Hearse" has taken over my #1 spot. But really, any of those three would be good bets.
I also think a lot of people would say "Ghost Reveries," but I probably wouldn't. That was my first Opeth album, but I couldn't get into it until after I got really into "Still Life." Then, I revisited "Ghost Reveries" and really liked it. (Ironically, my favorite Opeth song "Ghost of Perdition" is off of "Ghost Reveries," so I'd suggest listening to that despite whichever album you get.)