Let's go from the PS1 era to current times, since a lot of games from the NES period were
hardcore... like no checkpoints, one save spot in a dungeon in RPGs and stuff like that... so, what's the most difficult game you've played?
For me, it was Ninja Gaiden on the original Xbox. To further justify my case, it was voted by a few videogame magazines (when they existed) as the hardest game of the year when it came out.
Groups of enemies didn't "take turns" fighting you, and if you let your guard down for one second you'd get slaughtered. The first boss in the game was as difficult as the last boss in most other games. And I feel like I just got lucky beating the last boss after getting my ass spanked a dozen times.