That stupid course in Star Wars Racer where you have to stay on a very narrow ledge. When you fall off you're relegated to a super long detour and basically have to start over because you have no chance of winning if you fail even once.

Well, I've never beat Sans from Undertale, but if we're only counting those we did beat... Maybe it's Undyne the Undying from the same game. I'm deducing that from the time it took me, but to be honest there's a boss from another game that came to my mind first. See, neither of these bosses feel "unfairly" hard because you have to go out of your way to pick the hardest route to face them. There was one boss whose hardness seems unfair and bad, that was the third boss from the first Spectrobe game. This was really out of nowhere. The first boss took me a few tries because I wasn't used to he game mecanics yet, the second was easy, but the third? I had to look up strategies online, make dozens of trips between my ship and the bosse's location, fighting every mob on the way each time to level my spectrobes up in addition to digging up minerocs to feed them; got them from adult to evolved and finally then I could beat it. After this fight, my spectrobes honestly felt overleveled and every enemy from then on felt super easy.

Worst part is, the plot didn't even justify this boss to be so hard. It didn't make it feel important. It wasn't directly threatening a populated planet, it wasn't anounced before it appeared, it was just there, in the middle of the jungle. And your mission was to find a diamond. Not an important and powerful item that's key to saving the galaxy mind you, a dumb simple diamond, of which the only purpose is to be expensive. So why would you, Rallen, of the Nanairo cosmic patrol; charged with protecting the whole solar system, go look for it? Because some rich bastard has crucial information about the krawls and you need this information to go on, but he won't tell you unless you give him this diamond. BITCH, if there was any logic to the plot, I'd be pointing my canon beam at his head and screaming at him to tell me all he knows or else! How dare a human withhold info necessary to save humanity?

Oh, and if this diamond has no particular power, why is a powerful krawl guarding it? It's not! It's just standing on top of it by shear luck, because fuck you.

God Of War 2 Theseus gave me way more trouble than Ares on Titan difficulty.

There might have been a mechanic I was missing but you had barely enough mana to cast all of the required spells to beat it, at least with the pickups I had found, like literally if I missed more than one lightning bolt I had to restart, and some of his spells would instantly kill you. I don't recall the specifics of why it was giving me trouble beyond that but I do remember bashing my head against that wall for hours.

Never really had aa much trouble as that fight. Not in Souls, Bloodbourne, that fucking Theseus and his jerk friend minotaurs.

The shadow enemy in super smash bros brawl, cant beat that mf

So this is dumb but there is a Mummy game, as on the movie, I think on PS2. You could play through as either O'Connell (Brendan Fraser) or Imhotep (he was fun but pretty overpowered). I don't remember there being a difficulty setting but I could be wrong.

I beat it with Imhotep but never could beat it with O'Connell. I want to say the final boss was the Scorpion King. I tried so many times.

The only thing I can figure is I didn't load up on as much ammo, grenades, equipment, etc. as possible and never went back to try differently.

Inner Agent 3 from Splatoon 2: Octo Expansion

What an absolute nightmare. They play like an online cheater. It's just back to back sub and special weapons. There's almost no room for error. I gave up after like thirty tries because the reward just wasn't worth the stress and frustration.

It's been a long time since I played so my memory may not be totally accurate, but the final boss in fatal frame 2. I rage quit it because it was a one hit kill, and before you get to them, you had to walk down a long hallway avoiding other enemies. It was like 5 minutes of work to get to the final boss only for him to hit you once and you have to start all over. So infuriating. The rest of the game was so much fun.

Radagon and Elden Beast at rune level 1. The area of effect attacks and the tracking projectiles are brutal. I was able to get a strategy that clobbered Radagon in <1 minute without taking any hits but it required really intense concentration and timing. Not being able to practice just the Beast was the worst part, I hated having to slog through Radagon every single time to attempt it.

Also Sister Friede at level 1 is a strong contender. I don't remember which took me longer to complete.

The guy at the end of Hotline Miami was pretty tough.