DSII just has a large quantity of gank fights but if you know what you're doing, it's not so bad. I think only two bosses gave me trouble in the early game, the three Sentinels and the trout demon monster in Black Gulch. It's the bosses towards the end that were often more problematic for me and even then a lot of them were optional. My other issue was the silly Soul Level nonsense mechanic that made it nearly impossible to be summoned for me but made it easier for me to get invaded by players who were a few areas ahead of me. The few areas I had the most grief in were often due to invaders blocking my ability to progress and costing me souls. I think my biggest issue in DSII was that parry timing seemed to be changed a bit, especially the counter riposte which for some reason I could never get done right.

For me, I think Demon's Souls is still the hardest entry, the controls are more clunky, infusions are both more important and situational, and the level design is far more sadistic than the Dark Souls series. In terms of boss battles, I feel DS1 and Bloodborne have the most challenging so far. DSII has too many filler bosses and DS3's bosses have been surprisingly easier than what I'm used to in the series but that might change now that I'm towards the end and all I have left are three story bosses and like close to a dozen optional bosses who tend to be more challenging.

Update Time:
I think the Dancer of the Boreal Valley might be the hardest boss in the game so far. Despite having really good tells, she's pretty aggressive and has longer combos animations and range than I would care for. My biggest issue was that without an ember activated, her grab move in her first phase was a OHKO for me since I rarely put points into HP so a bad roll was all it took to end the fight sometimes. Her and Aldritch have taken the most tries in order to beat, but nothing too infuriating yet.

Reached Lothric Castle proper and explored the King's Garden or something. Met the Consumed King, and he was honestly not too bad outside of some high damage from the charge move and whip attack. The hardest part of the fight was chasing after him cause most of his attacks in his second phase kept launching him across the screen and trying to stay in front of him is too dangerous. A friend mentioned to hit the wall in the back of the area you pick up the Way of the Dragon emote and I found myself in the Untended Graves. I honestly had a tough time killing the Soul Arrow spamming invader than Champion Gundyr and I got to explore one of the eeriest locations in the whole game so far.

Looked up how to find the Archdragon Peak and went there, but called it a night at the first bonfire. I think I'll try to take down the Nameless King because I feel he'll be a nice litmus test for dealing with the DLC. I've seen videos of the Sister Friede fight and I can already say I am both dreading and anticipating it.