I like post rock and some of the pop music stuff like Marilin Manson, though I tend to find doom metal, black metal, or melancholy rock tend to be the darkest and most intense depending on what you are going for.
Stuff like Saturnus, Katatonia, Anathema, Candelmass, Novembers Doom, My Dying Bride, Shape of Despair, Paradise Lost, and Funeral always tends to always stay towards the top of my playlists. Anathema especially, as they cover the full gamut of deathdoom to doom to postprog to melancholy rock. The intensity of the emotion in Anathema is almost unbearable, yet it's also as perfect as I've ever heard.
Anathema - One Last Goodbye
Now, if by intensity you mean, brute force cold harsh violent manic chaos satan satan satan stuff, then I favor the 2nd wave norse black metal stuff like Enslaved, Immortal, and Emperor. I also like some of the weirder/more wacky stuff like Abigor. Also, most of these guys have a good sense of humor about their music, like, Abbath from Immortal could outdo Gwar at Gwar stuff.
Emperor - I am the Black Wizards
Finally, there is what many consider to be the ultimate in dark/depressing stuff, the Dperessive Sucicdal Black Metal genre, which I never really got into. I am sure there are some musicaly good examples, but, It sounds a bit too sanitized and theatrical for my tastes.
So, instead, here is the funniest black metal video I have ever seen, The Black Satans - The Satan of Hell
https://www.youtube.com/watch?v=XOibIxl3dLo