Some people just mentioned bosses so
Aztec Falcon from Mega Man Zero. I know this was literally just mentioned but I feel the need to mention it anyway because it took me literally MONTHS to finish it. I got the Mega Man Zero Collection one day, I was having fun, and then this guy happened.
His pattern is unpredictable, his attacks are hard to dodge, and eventually I just gave up. I tried again every once in a while, but it took me months until I managed to beat the guy.
So a lesson to anyone making a game: Don't make the first boss the hardest that's terrible and bad
Oh also for an actual level the whole coliseum world in Kingdom Hearts that was as far as I made it in that game because the boss is unfair as hell and the rest is extremely monotonous. It feels like a tutorial level at first with all the random challenges, and then there's the actual coliseum part which is just killing enemies by spamming attacks and hoping you win. It's bad.