Depends on definition of "dark" but here are some more favorites that are on permanent re-watch . They can be considered dark if you do a character study on the roles. But, these are more mainstream, so can be watched with significant other, kids, etc.
Payback (there are two versions. the one with Faye Dunaway as narrator is "darker")
The Limey - has the violence
The Departed - has the violence and the twist
Confidence - not necessarily dark, but has the twist
Usual Suspects - same as above.
Sexy Beast - same as above.
To throw out war movies, the top three Vietnam flicks have a lot of darkness (I saw all of these pre-teen I'm guessing)
Apocalypse Now (and the documentary "Hearts of Darkness - everyone almost went insane during the filming)
Full Metal Jacket (Kubrick - duh)
Platoon (Oliver Stone - duh)
And if we want to go to TV series, Ray Donovan will send you places.
My definition of "dark" is probably aligned with most posting in this thread. And someone said it earlier, horror/jump-scare/slasher =/= dark. I was part of the OG that saw Blair Witch before the gig was up and I don't need that in my life; especially since I am located close to where it was filmed. I did not like finding talismans in the woods as a yute. Cerebral darkness I can handle; horror, less so.