Give it time...
My first ones were BWP, Deliverance and Damnation in that order. Then I got Still Life and was hooked immediately. Then I got MAYH and it took me awhile to get used to it. I was really disappointed, but now I really like it.
Now when I go to grab a cd, it's almost always MAYH or SL. I think right now, SL is in my Jeep's cd player and MAYH is in my portable cd player next to my bed. They are both by far Opeth's best two albums.
Now the other two are a source of bickering on this board. Morningrise is enerally described as a depressing record, and I like it less than most, but it still has its highlights. Advent is a great opener. Night and the Silent Water is a cool song (even though the band hates it). I like Nectar, even though it gets bashed on the board a lot. Black Rose Immortal is an amazing 20 minute epic that I dream of seeing live in concert. And the closer, To Bid You Farewell is a very sweet ballad. All in all, a disorganized but great record.
Their debut, Orchid, is by far my least favorite. I know many who agree with me, but I also know many who feel that it is their best. I think that's because listeners of Opeth come from varied musical tastes and so different aspects of their sound appeal to them more. I, for example, come from a heavy-metal / progressive rock background, which means that the more melodic stuff appeals to me better than the straight death metal. If you are into death metal, you will like Orchid.
So, here's what I want you to do:
1) Lock yourself in a room with a stereo that has speakers bigger than you, a comfortable recliner, and a week's supply of cocoa-puffs and beef jerky (okay, you may want access to a bathroom...) and do nothing but LISTEN to MAYH for 7 straight days. Before you know it, you will be in love with it (or psychotic... or both

)
2) Leave the room and go to your local cd store to purchase Still Life (perhaps a shower and shave first, though!). Repeat step 1 with your newly acquired album.
3) Continue the process with the final two albums.
4) After 1 month, you will emerge a better person!
Good Luck! I wish you success in the Opeth rehabilitation program.