how about "Dark Lite" on Havok? it does have dark in the name after all... just kidding, its a great track but not "the darkest" by a long shot.

In thinking about dark acid tracks like Immersion's "In the Freeze" on Routemaster or Site Breaker's "Hash Fudge Dave" on SUF come to mind. These two are plenty moody and both have monstaa acid riffs that will make you want to molest the speaker cabinets