How hard a game is considered to be depends on who is playing it. A three-year-old tackling Zelda will struggle. But equally, a new-to-games-parents will find Mutant Mudds quickly gets beyond them. The games in this list are known for being difficult. They wear the difficulty as a badge of honour. "None shall pass," except with the will, time and belligerence to get good enough at this particular activity to beat the high bar the game sets.
This might be grappling with the flying mechanics in Rocket League, getting endlessly lost trying to find the next guardian in Shadow of the Colossus or coming up with the right tactic to get enough money for the ship you need in Elite. Of course, some of these games can be made easier, but to play them at their best is to ramp up the difficulty to max (crushing on The Last Of Us for example) and let them give you all they've got.
This list includes 485 games from the last 38 years, with 5,913 likes. They come from a range of different genres and play-styles and are all good games if you want to take on a mountainous challenge.