Dark Souls is the only one I have experience with, but I would definitely recommend it.
It has got a steep learning curve but I don't think it's as difficult, as such, as it's made out to be. In saying that I realise the learning curve is what makes it difficult but after about halfway through I started beating bosses after one or three tries, which just makes me think it stops throwing anything new at you (At least until the last boss... urgh.)
It's story is probably the most difficult thing in the game though, it does have one and it is interesting, but good god do you have to do your research to piece it together. It actually has got one of the most fascinating techniques in multiple endings I've ever come across. if you play through it once it can reveal a fairly standard good vs evil ending(it's quite literally light vs dark,) but if you play it again and choose a different route (a fairly specific route) the endings will remain the same, but it will give another explanation of events which drastically alters the morality and consequences behind your final choice... personally I played through it once and let the internet figure out the actual plot for me but it was very intriguing none the less.
Atmosphere is very nice, probably the best game at portraying a properly grim tone I've played. Combat it fun, level design is as varied as a bleak landscape can be... other people have explained this stuff before; it really is worth playing.
I've been intrigued by Dragons Dogma for a while though but haven't heard much about it, I also want Witcher 2 since I just got a computer that can handle it, but I'm waiting for a Steam sale. Never tried either of them yet.
