A boss you died to 1000 times, an enemy that still strikes fear into your heart every time you replay your favorite game, etc.
For me it’s the Blood Starved Beast from Bloodborne. I died to it probably over 100 times my first playthrough no matter how many times I replay the game I still get torn to shreds repeatedly. Honorable mention cazadors from New Vegas
Ruby Weapon from ff7
Also that first boss of the 2004 Ninja Gaiden that’s supposed to be a tutorial boss lol. My god that was a game you die in A LOT. Souls before the souls games. It really forced you to learn the game properly, learn your moveset and to play properly.