What bosses do you consider unfair?

Personally, none,

Sometimes players claim the game is unfair because they are not skilled enough, which is fine. The game gives you a huge amount of options to make this difficult game easier for you, like mimic tier, armour, talismans, Buff Stacking, weapon upgrades, Bleed etc., which makes the game a lot easier.

None of them are cheating or unfair, except for maybe Buff Stacking. With proper Buff Stacking, you can seriously one-shot PCR.

The Scadu Tree Avatar can easily be dispatched by using fire magic, yet I still see many players who are so bitter about it. Do you know how I defeated the Scadu tree Avatar? I just kept spamming flame spells at it. Nothing else. It felt like I was cheesing it.

Then there are some truly powerful bosses, like Malenia, Promised Consort Radahn. They are extremely powerful and hard.

Are they unfair as a boss? No.

Here's my noob tactic that I myself use. This is not even a tactic used by the pros:

1. Pick up your shield.

2. Block the first barrage of her waterfall dance.

3. Do a roll to get away from the second barrage.

4. Do another roll to get away from her third barrage.

An extremely useful piece of advice: record your gameplay and check why exactly you died. Sometimes this reveals glaring mistakes in your gameplay that you do not notice when you are busy playing and enjoying the game.

At the end of the day, you, the player, are the one who is truly unfair. Imagine a boss who has 14 red flasks+12 for healing, who will heal from each attack, who will use scarlet rot, who will use talisman, who will use the best armor and forever revive themselves and come back to challenge you after you have rested to full health, who can use buff stacking, and make themselves so powerful that they one shot PCM. Does that sound fair? That's you if you know what you're doing.