I didn't have too much trouble with any DS3 boss; the fight that gave me the most trouble was the Crystal Sage. I don't know what I was doing wrong there, but I kept getting staggered and killed. It didn't really feel challenging though; more like I was just doing something wrong. The Dancer was definitely the hardest fight that felt like I was losing because I wasn't good enough.
I beat Champion Gundyr and the Nameless King on my first try, but I had prepared for the Nameless King and specced my character out for the fight (upgraded Knight Lothric Shield, Thunder Stoneplate Ring, lightning resist armor, etc.).
Deacons of the Deep and the Dragonslayer Armor took me a few tries each.
I beat the Twin Princes with a real person's help. I summoned an AI helper for Pontiff Sulyvahn, Soul of Cinder, and Old Demon King, but mainly for quest/equipment set purposes. I don't think they served too much help beyond distraction.
Champion Gundyr is definitely my favorite boss in the game, though. Super fun to fight with how fast and aggressive he is.