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
Burt the Bashful from Yoshi’s Island. Not because he was particularly difficult but because he is such a ridiculously silly boss in a game full of them and I still sometimes think about Burt and chuckle out loud in front of people.