On break now, so I can type a bit more -
I've been a huge fan of the series since shortly after Demon's Souls came out in '09. I had read reviews, and my friend got it, so I figured I'd buy it myself and I absolutely fell in love with it. Beat it about four times, I think. I replayed both it and Dark Souls before playing Dark Souls 2 last month, and it still holds up amazingly well. The atmosphere is just sublime, and while the gameplay doesn't have the new features the other games introduced, it didn't feel like a step down when coming back to it. It's my favorite game.
I bought Dark Souls at launch on PS3, and was extremely hyped for it. I found Demon's to be a bit more aesthetically pleasing and consistent in quality, but despite that I still loved the game. I beat it once on PS3, bought it and replayed it on PC a year or two later, and replayed it for a third time last month. It's grown on me more since then - I prefer it's lore over Demon's and I think the Artorias DLC is the best the series has to offer. There are also some places that are really beautiful, like the Crystal Cave and Anor Londo. Like its predecessor it's undoubtedly a masterpiece in my mind.
Dark Souls II's development background shows in its gameplay. It is not a Souls game by the Souls guys. It's just, sadly, not. It's a good, fun game, but it's exceedingly clear that it was a game made by people trying to emulate other people's work without truly understanding what made it so good. I remember what my girlfriend said when she first started playing it (Dark Souls was one of the first console games she's ever beaten, and her favorite game now after beating it a few weeks ago) - "When I die in this game it's not the same. I always feel like I'm saying 'that's bullshit' unlike Dark Souls 1 where I almost always felt like it was my fault". This is of course a common complaint, but it really spoke to me that she, who doesn't even play games, was instantly able to feel a difference between the two. I feel the same. Enemies with stupid high poise, extremely accurate tracking, and infinite stamina? Why? Difficulty is often a result of bad enemy designs or crowded mobs in a cramped area. It just isn't as fun.
I've beaten Dark Souls 2 once already, and am doing a co-op run with my girlfriend now. It's really fun co-oping, but as a solo single player game the difference is extraordinarily vast. Demon's and Dark are both fakery very close to each other and beat the other in specific categories, but Dark 2 is worse than both by a huge margin.