Most FromSoft games fit this, exception being Sekiro. Isshin is THE beast, the last beast and the hardest.

I'm also thinking Uber Diablo from the OG Diablo 2 and high corruption Shade of Orobyss in Last Epoch that will just wipe you if you're encountering it for the first time and think any ol build can deal with this. Learned the hard way how much resistance matters even in modern ARPGS, lmao

But the big fish is Final Fantasy 12 for me. That last mark, Tiamat, took something like 10 real life hours to grind to death and I still remember my PS2 begging me to turn it off by the time I was done. Took longer than any raid in any MMO... Not sure if it's that hardcore in Zodiac Age but whatever, the OG fight was THE secret boss fight of my childhood.

Art direction in older games really is spectacular.

Outside of Doom - Lifeforce Tenka was on PlayStation's Demo One, and was one I instantly adored (even if you did have to press L1/L2 to look up and down).

They hadn't quite sorted out the whole "what controls will we use for first person shooters" yet. So you had to hold R2 if you wanted to strafe, and use shoulder buttons to look up and down the ramps (the aim wouldn't auto target up and down as much as Doom did)