Actually Joe I'd nominate any level past the first one on Ninja Gaiden for NES. I have bite marks on my NES controller from that game. I beat it once, never went back.
I still have not forgiven that game, and probably never will. Strangely, I still rate it an 8 out of 10. To be honest, though, I'm not a big fan of the Ninja Gaiden series. I wasn't all too impressed with the new stuff and I don't care for the arcade beat 'em up. The first two on NES are the only ones I really care for.
As a kid I hated that ! I've never pleayed the game as an adult for all I know I could do it rather easily now, but it took over a year to do that level.
My sis then deleted the save files and, I kid you not, I cried.
Similar thing happened to me when I played Legend of Zelda: Link's Awakening. I was really close to the end of the game and my mom let my niece play it. She deleted my file. Not nearly as aggravating as your situation, but still disheartening starting over.
Ghosts 'n Goblins Third level can't make it past the multiple gargoyles and still have never done it.
THe GNG series is another one that only has select games that I care for. By "Select games," I really mean "one game on the Sega Genesis."
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I also seem to remember there being a really irritating dungeon in Beyond the Beyond that had invisible holes everywhere. I swear to god that dungeon took me an entire week to complete. So not worth it, too, as that game is horrible.