Dark Souls 3 was the first Souls game where I didn't get stuck on a non-DLC boss for hours.
I chalk that up to having played all the previous games multiple times with different builds on a couple of NG+ cycles.
My first time through Demon's Souls was the toughest for the entire series. 1-1 took hours just to get to Phalanx. I stupidly spent hours in 5-1 after I beat 1-1 without even getting close to killing the Leechmonger there and deciding on just going to 2-1 instead. 4-1 was also a brick wall because of the flying manta rays and gold skeletons. Old Hero pummeled me for an hour. Maneaters embarrassed me for a day's worth of a gaming session.
Having gone back to it after I had beaten DS1, DS2:SOTFS, and BB multiple times, it's the easiest, unsurprisingly.
DS1 the first time was easier than DeS the first time. Still had a lot of trouble with Bed of Chaos, Manus, and Gwyn. I still get triggered at people saying Gwyn is a perfect "theme" boss because of how easy he is when you parry him. I didn't know about that strategy at all, and my god was he a relentless monster on that first playthrough! Took me more than an hour, but I eventually mastered dodging just about every single attack of his.
Played BB after DS1. First third of the game was rough. Just fighting my way through Central Yharnam was super humbling. Blood-starved Beast kept me up until 3am without me killing it. The game definitely got smoother later on though. The one roadblock in the main game that continues to be a challenge to this day is the Bloody Crow of Cainhurst. That one hunter may as well be a proper boss with a named lifebar.
Then I played the Old Hunters. Ludwig and Orphan are nightmares, even on replays. And yes, the two giant shark enemies in the well rank way up high as the toughest non-boss encounter in the series.
Chalice dungeon bosses weren't that bad. Didn't take too long for me to memorize the Watchdog's attack patterns in the Defiled dungeon either for a clean run.
DS2: SOFTS was child's play after Bloodborne. The only non-DLC bosses that gave me some trouble were Darklurker and the Ancient Dragon. I do think that if I never had to fight my way in that hellpit of an area just to get to Darklurker, the fight itself wouldn't have been so tough because I would have had way more estus. Ancient Dragon was just tedious because of its ridiculously large health pool. That dumb one-shot AOE flame breath attack also didn't help. One mistake in positioning, and it's over.
I actually didn't have too tough a time with the DS2 DLC bosses. Found traversing the levels to be more challenging actually! It was just the Lud and Zallen fight and the gank squad that took some time. Doing those battles solo required a loooooot of running and evading.
And going back to DS3, it was just the Nameless King that gave me trouble, but not to the extent that others talk about it. Never had any issues with the first phase either. Was pretty used to manipulating the camera by then.
Sister Friede x3 might be my favorite Souls boss battle period. It's not easy, that's for sure, but it never felt headbangingly difficult either. Like that one poster said, the first phase is trivial when you figure out she always jumps straight behind you when she goes invisible. Very easy to stunlock her to oblivion. Second phase, I found focusing on Ariandel to be the simplest way to get through it. Just backstab Friede when she tries to heal. That third phase for me is the only truly hard part on replays. Still super fun!
I thought the Demon Princes were going to eat me alive for a long time, but once I got into a rhythm with drawing aggro from the right bat at the right time, that phase went relatively quick. The second phase was still a problem the first time through the DLC, but not much so the second time.
Gael's got a flow to him that I can either ride to a smooth victory or get knocked off of for a quick defeat.
Midir is in the top 5 for toughest boss, because much like the Ancient Dragon, it has an insane amount of health and can easily one-shot you if you fuck up once no matter what your build. I think I read somewhere that it has the highest HP of all Souls bosses. Unlike Ancient Dragon though, I find Midir to be way more fun to fight. Also, holy shit does the Wolf Knight Greatsword fuck his face up real good. Took me by surprise how fast I killed him with it!