Ive already said this in another thread like this one but Cave make some of the hardest bullet hell shmups especially as many of their games require you to beat a second loop of the game (thats much harder thean the 1st loop), all without using a continue, and if you do that you get to fight a hidden boss.
My personal pick as the hardest goes to Dodonpachi Death Label on the PS2, this is a special boss rush of the main game and you have to fight all the bosses from the 2 loops, and the final boss Hibachi isnt just alone you must fight two of them at the same time.
You get hit once and its game over. Suffice to say (iirc) that only one person has managed this feat of skill 7 years after the game was released.
Here is the vid of the guy beating hibachi
On the subject of difficulty in games there is 2 types imo, 1 type is how many old games use difficulty where the player has zero chance to beating the challenge on their first play through EG the jetbike stage in battletoads and I wanna be the guy.
The 2nd type is the hard but fair type where the player can beat the game first playthough if they have the skill to do so. Imo more modern games use this type of difficulty even bullet hell shmups are like this (eg some of the best CAVE players on shmups forum have boasted theyve beaten a Touhou game or two in their 1st playthough)
There is a 3rd type of difficulty, that of when the game is broken in someway or has shitty controls. But im sure we all can agree these type of games dont really count.