Demon's Souls, but I have yet to play Dark Souls

I would actually have to say New Super Mario Bros. Wii. I always have trouble with that game. :/ It can get quite tough! Very, very well made game though, fun too! Just really had for noobish me. XD

The easy choice would be the Demon's/Dark Souls tandem, but then, if you consider game trials and challenges, I remember both Mirror's Edge and Donkey Kong Country Returns to be bloody challenging/frustrating/addictive.
Then you could also consider difficulty regarding how hard a game is to platinum, which I haven't tried and therefore, wouldn't know either.

Super Meat boy if you get all the bandages and beat all the warp zones and the special stupid hard levels you unlock when you get all the bandages. Still not done with that bad boy.

Killzone 2 on elite mode, and I beat it by the way!

MGS 4. It challenged me to see how long I could put up with Kojima's nonsense.

Demon's Souls

Prototype the game is so bad it was a challenge playing it.(Have yet to finish it)

Ninja Gaiden 2(360) I couldn't image playing this game on the hardest or even second hardest difficulty.

Count me in on DKCR. Dark souls was easy compared to trying to get through DKCR in co-op. We gave up on it.