best is obviously relative....but if you havent heard the band before...but it seems youre not new to heavy stuff...so id have to agree that still life or my arms your hearse is a good place to start.
I also listened to Deliverance. I didn't like it as much as Black Water Park. I was actually thinking of buying My Arms, Your Hearse but i wanted a second opinion
The only other acceptable answers are Morningrise, or Orchid. All others (besides deilverance, which was actually not really that good) are amazing, but not the best.
my faves are BWP and MAYH... i would start there... then Still Life, and round it out with Morningrise, Orchid and Deliverance/Damnation in no particular order.
Deliverance, to me at least, has more enjoyable parts, but all the repetition and sort of sameiness I think causes it to be a less complete work as a whole than Still Life. In other words, though I listen to the song 'Deliverance' probably once a week, I won't sit and listen to the whole album anywhere near as often as I do Still Life.
Use Blackwater Park as a test. If you enjoy it, things will only get better as you move backward in the catalogue. And once you develop the taste for them, then listen to Deliverance. Damnation is optional, as it's mostly prog, and not a full representation of Opeth's sound.