Hardest, hm...good question. The first game that came to mind for me was Demons Souls, but that was literally just Flamelurker - the rest of the game was disappointingly easy. One hard boss does not make a hard game.
Resonance of Fate, perhaps. There was a game that liked to screw with you. Getting into Critical Condition was effectively a death sentence - and with three bezels to start it was extremely common for that to happen if you didn't meticulously plan your moves - and the game itself was erratic, so difficulty never really evened out: it was brutal right from the start, and would occasionally lull you into a false sense of security only to screw you over again a few moments later in the next chapter.
All too frequently you'd have one or even two party members taken away and would have to fight bosses undermanned, your level was tied to your three weapons, and your weapon's power was tied to customisation...and in order to customise sufficiently, you needed to increase your level to increase your equipment load. You were never quite powerful enough throughout the game to really breeze through it, and with enemies respawning in areas you'd cleared on some maps, getting through could be a really tough time, and if you couldn't get through, getting back was a nightmare.
I ragequit the first time on the fourth chapter, because it was an escort mission. In a game like this! You had to escort a constantly moving mine cart with a statue on it through several levels of marionettes with flamethrowers and chainsaws, culminating in a boss that filled up well over half the screen with the mine cart STILL MOVING. This was BEFORE you could make grenades and ammunition types, too - you unlock that particular ability in the next chapter. So resources are very scarce. You could actually stop the mine cart from moving by putting a character in front of it, but enemies would still move towards it, and being a character down - because you could not move them AT ALL or the mine cart would start moving again - made clearing the map out take that much longer.
Enemies had body parts you'd need to break through, too. You'd also need to attack them twice - machine guns to turn their health bar blue with scratch damage, then handguns to deal actual damage. Scratch damage also regenerated with time, and it was absurdly fast for some enemies and bosses.
I've platinumed the game twice now on different accounts and I love it. But I was stuck on that chapter for something like 3 years.
