I would have to say Conquest of the Crystal Palace (NES). If you like NES platformers in the vein of Ghosts & Goblins (ninja action vs. plumber action) then it's a great title. It's by Asmik which was a great developer - Boomer's Adventure in Asmik World is one of the greatest Gameboy action/puzzle games ever. However, this game just gets excrutiatingly hard in places, not unlike G&G. There are places where you experience slowdown, for example, because there are 20 enemies falling from the sky that you have to slash your way through like you're some sort of demented snow-shoveller in a blizzard or something. Then, even after all of the difficult levels, if you reach the end, the boss is 10 times harder than the rest of the game. A true "boss," in other words. Not only content to shoot at you from the right side of the screen, he also rains down the ceiling upon you. And if you've made it that far, your lives and continues are gone so if you want to learn the boss' pattern you'll have to play all over again. I actually have this cart for NES; it's a great and rare classic. Now that I am "old," I can hardly get to the first boss let alone past him.
Another title I will mention is Blood for PC on the hardest level. If you read the instruction booklet, the 4th level of difficulty is only meant for co-op play, but you can still choose it in single-player mode. I had an extremely difficult, long journey to beat the game on the hardest mode using every cheat except God mode. What's more difficult than insane, evil mimes after all, while you're just trying to chill and play a game of zombie-head soccer?
Jam it back in, in the dark.
85242
35212
speculative