Aliens on the C64. I had the budget version which, unknown to me until 20 years later, was missing the printed map you would get in the full price version.
An early predecessor to the 3d fps, it was ******* terrifying, as well as difficult. Might go back to it with emulator. and with the map printed out.
Loads of C64 games were seriously hard. They would otherwise be so short that I guess they had to make them that way.
I tried playing Mega Drive Lion King the other day after completing it 23 years ago on Amiga. I couldn't do it. Old games punish you and are utterly infuriating
I completed the SNES one in a weekend, having borrowed it from a friend. But in those days there tended to be differences between the consoles, so MD one might very well have been harder.
Don't think I ever finished Jet Set Willy also.
Depending on which system you were playing, it might have actually been impossible to complete. The C64 version was terminally bugged such that you jumped further going left than right (or vice versa), meaning there was a section (or sections) that simply could not be done.
Oh, and if I recall correctly, the ending sequence wasn't even coded into the game!