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
Some of the endgame interceptor/gauntlet/hot pursuit ones were no joke too
But nothing comes close to the endgame time trial stuff
“Contact sport” is another difficult one
I haven’t played hot pursuit in ages but I can still remember those stages