1) Ehhh. If you play fair with Gwyn, or if you are a straight sorcerer, then yeah, you're going to have a bad time. But the arena you fight with him in is easily exploitable, and that jump kick he does when you are at a distance, is easily punishable.
Four Kings I considered way harder. If you fight them with anything other than a high vitality, then their grab attack will basically insta-kill you, and it's easily triggered if you're standing close to them after their whirl combo. And if you can't deal out enough DPS, then you can easily get swarmed by way more than just 4 of them. It's a deceptive battle, and you actually have to think to get through them.
2) Soul of Cinder is a fucking pushover. All of his attacks are easily learned, and it's almost impossible to get combo-locked by him. Don't get hit by his sorceries, learn how to roll, and don't get greedy, especially when he transitions into an AoE attack.
Oceiros was way more frustrating. He takes a while to get his moveset down, and when he starts rampaging, it's over if he hits you and you haven't been min-maxing your stats. Not to mention that his magic defense is insane, and summoning people tends to make this boss way harder, since summons frequently die to him, and leave you with a much higher health pool.
3) Dancer is a given. Especially if you're like me, and you're running a SL1 build. If you don't have the stamina, then she is your worst damn nightmare.
4) Fume Knight is a breeze once you learn his moveset, which isn't hard, either. If you are dumb enough to go into the battle with more than one statue still intact, then... that's your own damn fault for letting the boss get in free heals.
The Ivory King is way harder. If you don't have the DPS, then you're fighting the Ivory King with a bunch of adds by the time he arrives, and having any summons will give the guy way more health than he knows what to do with. Especially with his insane poise levels, since the guy is almost impossible to stagger, and he can easily hit you through your attacks.
5) Manus is definitely a tough one. He's actually one of the very few bosses that allowed summons, where I had to have a summon before I was able to beat him for the first time. Sif was a real life saver, and I had serious issues in NG++, where Sif dies super quick, and becomes largely useless, even as a distraction.
6) Artorias is the only boss that took me over 20 tries before I was able to beat him. Now that I've been playing the Dark Souls games for years now, and I've fought and beaten him numerous time, he has finally gotten easy. But I legitimately had to stop playing Dark Souls for a few weeks, because of how badly he frustrated me.
7) I honestly don't understand why people have issues with the Nameless King. I mean, I do, because players have a tendency to not properly prepare for tough bosses, even after they've died to them a few times and should know better. But with proper lightning/fire defense, which the Dragonslayer Great Shield is amazing for, once you've learned to time your rolls so while you're working for the critical attack opportunity you can dodge the occasional spear thrust, and once you've learned to stop standing in the middle of the dragon's damn fire attack, he's really not so bad.
Once he's by himself, he projects his attacks, and if you get caught in them, then you need to get better at your timing. He's honestly not that bad.
8) Smough & Ornstein are some of the most fun bosses in the Dark Souls games. That being said, they aren't hard - they are actually some of the most fair bosses in the entire series, too. There's plenty of time to focus on a single boss, as they frequently split up, giving you chances to strafe between pillars and avoid getting ganged up on. Camera is actually the largest challenge in this fight.
Summon another player and/or Solaire, and the fight becomes trivial. Once their attention is entirely separated, and especially if you are elementally buffing your weapons appropriately, their health pools go down very quickly.