I voted for Still Live, but it was going to be a tossup between Still Life and Morningrise.
Orchid is the first one I got, and after a while, it really grew on me. Then MAYH came out, and I really wasn't that impressed, and even after several listens, it's my least favorite Opeth CD. (but I do like it quite a bit.) I guess I was a bit bummed out by the shorter songs. I eventually got Morningrise, and for a while, I kept thinking, "It's not as good as Orchid" but it eventually became my favorite one. The thing is, I can listen to any of Opeth's CDs over and over again, so none of them are "bad" by any stretch of the imagination.
I spent a year trying to order Still Life from CD now, but it was always a $20+ import that was never in stock, and I didn't end up getting it until about a week before BWP was released. After a couple listens, Still Life had totally blown me away with the quality of the songs and music. BWP I wasn't as impressed with (remember, I got them both almost at the same time) but it's started growing on me, and it's definitely a great album. Heavier than Still Life, but it's still full of lots of great music, and in some ways it's better than Still Life, but overall, Still life takes the prize as my favorite.