Thunderblight was the first boss I fought in BOTW and I did it with 5 hearts. He was a pain in the a$$.  EDIT: The rest of the bosses were a big let down (still haven't taken on Gannon) after that fight since I had enough hearts not to get 1 hit and had better armor and weapons.  I only had the mastersword for the last of the devine beasts and it was way too easy.

Spoiler warning!

The worst boss fight I can remember though was Metroid Prime (gamecube version if there is a difference). I wasn't using any guides and had a hard enough time with the spider form. Then it went into the core form and would seem to disappear, kill me and I'd be right back at the beginning with the damn spider form. I was very frustrated until I figured out to switch up visors on the final form. I still think I only ever beat that boss once.

If you had a tough time with Thunderblight, don't touch any games pre 2005 or so. They will wipe the floor with you.

The secret Shadow Jago boss fight from Killer Instinct (2013) rekt me consistently, but secret fighting game bosses have a history of being incredibly difficult, so that's probably not too surprising. Pontiff Sulyvahn (Dark Souls 2) was another one that gave me a ton of trouble. Ultimately, though, I'd probably go with Kaptain K. Rool from DKC2. Only being able to take 2 hits, the required pattern memorization, and the sheer amount of health the boss had; that one kicked my ass.

Some of the boss fights from Devil May Cry 1 and 3 were incredibly difficult as well.

whatever that roided up guy at the end of Metal Gear Rising was called

i haven't had to give up on a final boss since I was a kid, but that dick ruined that run

No luck. I already beat him. Beat the whole game, actually. 

First phase, just back flip after he warps towards you. Slash, slash, slash. Second phase, pick up the things he drops onto the ground using the magnet. He'll shock himself. Then, repeat the first phase.

That's not the point. Thunderblight is an unbalanced mess. Coulped with a laggy boss room. Compared to all the other bosses. Including Gannon's two forms. I'm not saying I want the game to be easy. Like the other 4 bosses. But, Thunderblights difficulty came out of nowhere. So then i was like: Okay, I'll properly stock healing, get weapons, shields up the ass, for Death mountain. I killed that boss, in 3 minutes. I was like WTF... I fought all 4 divines without the Master Sword. There's issues with bad camera angles. Him falling onto the top platform. Weird issues with Double handed sword weapons, not registering all the time. Lightning Rod missing etc. It felt like he was cheating.

Sigma from Megaman X. I don't know why I had such a hard time with him. It was horrible. I know I had issues with Wario. In Super Mario Land 2. When I was younger.

Not a boss but did anyone want to break things because of The Apprentice in Soul Calibur?

First boss in mario made me rage quit

I thought the Omega Pirate from Metroid Prime was a jerk. Took a few tries to beat him, but the real kicker was the length of the encounters. It was a test of endurance.

The final boss of Grim Dawn during late early access was the worst. The boss kept spawning overpowered minions and spamming powerful ranged attacks all the time. I think I died more than 30 times while killing him and I couldn't have done it if his health regenerated when I died. Luckily in games like Grim Dawn, enemy health regeneration upon death isn't common. And as luck would be, the boss got nerfed the next day or the day after that. So yeah, good times (not).

Also, not exactly a boss, but sort of. In Driver 2, in the third last mission, you have to chase a guy and destroy his car. The mission happens mostly on a mountain road where it's ridiculously easy to fall off if you try too hard - which you do a lot, since the target drives pretty fast. And like in every mission, it's also possible to wreck your own car, which also happens quite often. It's a pretty simple mission but it took me literally years to beat it, cheats or not. I think I finally did it without, but the mission was ridiculously difficult.

To me, a good boss is a boss where, if you figure out how to beat them and you have to tools, you can win. A bad boss (or should I say frustrating) is a boss where you know what to do and the game won't let you do it. Or the weakness is so obscure, nobody could figure it out. Or it just doesn't play by the rules.