Opeth's most depresive song......

Tough choice, because while songs may have melancholy, sad, tender, or depressive parts, most Opeth songs have more than one emotion or mood throughout. Dirge for November, Night & Silent Water, Twilight was Robe, Bid you Farewell, Fair Judgement, Rid the Disease, have some of the most depressive parts, but also some uplighting, eerie, angry moments.
 
To Bid You Farewell
In My Time of Need

But ultimately, if you want to count it, the Demon/Credence combo. In context with the story, that's the most depressing thing I've heard from Opeth.
 
Since when is Opeth depressing?
Seriously, I don't expierience Opeth's music as being depressive. To me Opeth kinda creates soundscapes, just lay down and let them take you to another world. If you want depressive listen to My dying bride, that's depressive!