Hard to define "darkest". Eventually you get to the point where after pouring a glass and holding up to a light none will come through.
Generally speaking, I guess I would consider Imperial Stouts to be the "darkest" beers. The trademark of that style is being dark colored, thick, richly flavored, and high ABV.
Also keep in mind darker does not mean higher ABV! I've had plenty of beers that are as pale as Budweiser that would knock your socks off! Likewise, a beer like Guinness is actually pretty low ABV, something like 4% or 5%.