There's been many countdowns for the hardest video games ever made. Many of them include popular games such as Contra, Metal Slug, Battletoads, Ikaruga, R-Type, Ghouls 'n Ghosts and Devil May Cry.
Here's a list of video games which I've played that I consider extremely hard, and at times, harder than the games mentioned above in no particular order:
Odama (GameCube): This game was developed by the same people who made Seaman: Vivarium Inc. Odama combines the elements of pinball, Real Time Strategy and voice recognition. You control a gigantic ball called the Odama with your left and right triggers, and you can move the left analog stick to influence the way the Odama moves. The Odama can buildings, enemy soldiers and your own as well, so the player is forced to carefully maneuver the Odama in order to make it to the next level. The player also controls a set of soldiers who are controlled by shouting 11 distinct commands into the microphone to make them move, defend, rally or to press forward against enemies. There are also a few soldiers who carry a giant bell. If the Odama strikes the bell, the soundwaves generated by the bell can take down both enemy and friendly soldiers. However, if the Odama hits the bell when it's glowing, it will turn green, and make enemy soldiers hit by it fight on your side, and your soldiers remain unharmed. The objective of each level is to take the bell to the other side of the level within the time limit while surviving and not being pushed back to far or being defeated. The game is extremely stressful and unforgiving while your solders lessen, the time gets lower, knowing what to shout into the microphone and the Odama causes more harm than good. There was a rumor that an easier version would come on the Wii, but this never happened.
Cubivore (GameCube): Originally intended for Nintendo's failed Nintendo 64 DD, it was ported to the GameCube. The art style is extremely weird and crude because everything is literally shaped like a box, even the sun, moon and clouds. The world has been taken over by a killer cubivore, and it is your job to go up the chain of evolution to become strong enough to defeat him and his lackeys. This is done through eating other animals and taking their meat to make yourself stronger and defeating boss cubivores and taking their powers. Strangely, this game has elements of Darwin's theory of evolution. With the cubivores devoured, the player must determine which form is the best to use. Some may jump higher, do more damage, run faster, etc. Unfortunately, the game is actually too complicated for my taste. With all the different meat types and the fact the player must go through 150 mutations, plus the fact that enemies have unforgiving difficulty, this game can be extremely repetitive.
Bangai-O Spirits (DS): This is a sequel to Bangai-O on the N64 and Dreamcast. There are a large number of puzzles and challenges in this one. The player controls a tiny robot that can do many things from fire bombs, bouncing balls, missiles, and can use melee weapons such as baseball bat and a sword. Each item has strengths and weaknesses. This robot makes Gundam Wing look like Hello Kitty. The game can actually slow down because of the sheer number of objects on the screen. Many times, there will be over 400 bombs with enemies firing back and the game will slow down while processing each one. This issue is notorious in the Bangai-O series. In many levels, enemies attack immediately and death occurs in less than 5 seconds if you don't react fast enough. Also, some levels need to be completed in a specific order or else the player can get stuck. The DS version has a strange feature called "Sound Load" which can create and receive sound waves through the microphone to store levels. This is similar to the way a 56k modem works. This means that I can go to YouTube and play sound load videos on my desktop and have my DS listen to the sound to store the level. I tried this many times, and only got it to work once. This game is for hardcore gamers only.
The Haunting (Genesis): You play a ghost who is trying to scare a family out of every home they move into. You do this by possessing objects in the house such as making walls bleed, turning a couch into a monster with blood dripping, making plates fly in the air, etc. The player's health is represented by slime which slowly drains. If the slime is completely gone, the player must go through a dungeon level to recover slime, and it will be game over if the player dies too many times. Scaring family members out of rooms in the house makes slime fall from the ceiling which the player can take. With every level, the game gets increasingly more difficult because the player is attacked by other demons, the dog's bark can cause damage and the player must perform more scare tactics in order to stay alive. There is no save options, and the player will always start from the very beginning should they choose to restart.
Heavenly Guardian (Wii): Originally supposed to be a sequel to Pocky & Rocky on the SNES, it was renamed to "Heavenly Guardian" and released on the PS2 and Wii. They player controls a girl who can fire different projectiles depending on where the Wii remote is pointing. Shaking the remote triggers a blizzard which freezes all enemies which can be destroyed for bonus points. This game becomes difficult because there are no save points, it's difficult to gain health and lives after they're lost, and enemies attack in weird patterns that are difficult to dodge. For example, one enemy attacks in 3 random directions at once and sometimes fires 1 at a time. With the multiple enemies, low health and the lack of a save system, the player must anticipate enemy attack with every step.