I would have to say Akerfeldt because, to me, Opeth has the most listenable material. Miakel's vocals, harsh and clean, make the music more listenable, such as the song blackwater park. He adds some much intensity to the songs on Blackwater Park and other albums.
But Daniel has the widest range and is clearly the best all range singer in music period. I really like alot off of Remedy Lane, Perfect Element Pt. 1 and also Entropia. He is a great singer who adds alot of emotion and feeling, not to say that Akerfeldt doesn't though.
That really is a hard question to answer, but if i had to choose it would be Akerfeldt for sure.