I admit that I'm not the gamer I was in the 80s but I used to blast through what were considered some of the hardest NES and SNES games of all time in a weekend rental.
Cuphead kicked my ass. I think I got halfway through it. But again, I'm not the gamer I used to be.
There's a few NES games that elude me that I've wanted to go back and 'get my revenge' on. Things like Ninja Gaiden or Battletoads. I've beaten Tyson, Blaster Master, Double Dragon, all the MM games, and so on.
Without throwing anyone under the bus that used their free time for other things, I haven't lost my step thankfully. It's gotta be the right kind of challenge though. I want to feel like I'm making progress for my effort. If there's a brick wall then I don't find that fun anymore because I have a ton of other games to play.