Heihachi in the first Tekken is much harder and faster than his later incarnations, due to being the boss of the game. This has been written into canon with Tekken Tag 2 supposedly featuring a return of his younger self, when he was much tougher.
Tekken 5: Jinpachi Mishima (unavailable to players), who can interrupt any attack you throw at him and possesses a fireball that takes away two-thirds of your health with a single hit (which he can also fire rapidly in case you try to sidestep it). Yes, the final boss of a hand-to-hand fighting game is best known for his nigh-unavoidable magic stun and long-range fireballs. In addition, he can heal himself by stealing your life bar and phase around the stage so not only can he make it impossible to hit him if he so wishes, it is possible you may hit him and still get a "perfect" loss.
If you are jumping in the air, the stun will drop you down to the floor and you will reel from it. If you are as far away from Jinpachi as the game will allow, the stun will hit you and you are unable to move. If you are blocking, you are stunned and unable to move. If you are on the floor, the stun will hit you, make you stand, and have you reel.
Also, there's one thing that gets you no matter what - The stun/fireball combo. Go go, random stun counter followed by avoid-this-or-die fireball. Both mechanics are fine on their own, but when combined, there's nothing you can do.
Ask any Tekken player you know how bad would it be if you couldn't move for six seconds, given that Tekken's claim to fame is juggling and combo-ability. Then add to the fact the weakest of Jinpachi's moves will take off a fifth of your lifebar and is combo-able. And you have the equation for a true SNK Boss.
Azazel, the literal Dragon of Tekken 6, is much worse than Jinpachi. He's got Devil's Eye Beams, Jinpachi's stun, plus even more ridiculous moves such as raising boulders out of the ground, throwing barrages of ice projectiles, and the ability to regenerate. Not to mention he doesn't even stagger on normal basis, and when it happens, he takes no damage. May be the Grand Champion of SNK Bosses.
The Bonus Boss NANCY-MI847J has ten times the energy bar of other fighters, cannot be stunned or juggled, attacks with lasers, machine guns, and missiles, and has an Eye Beams attack that cuts holes in the floor, killing you instantly if you're standing there.
Boss Jin, whom you fight before Azazel, and right after Nancy. At the start of the round he automatically activates Rage Mode, meaning he deals twice the amount of normal damage. Not only that, but his AI suddenly increases to that of an expert player, no matter if you're fighting the CPU or a ghost, and will counter your strategies at every turn.
Tekken Tag Tournament 2 gives us a three-stage Boss Rush featuring the return of young Heihachi and Jinpachi Mishima at Stage 7, Ogre at Stage 8, and finally fan favorite Jun Kazama for the Final Stage.
The first two stages are warm-ups for the battle against the latter. She starts off as a defensive beast, completely at home in blocking most of your attacks. Take into account that she had been spending the game learning your moves and tactics and this is made even worse. She also uses her Attack Reversal every now and then, which is considerably rare for Tekken AI. Since she is a solo fighter, her entire health bar is doubled and must be depleted to knock her out, including her recoverable health. Once you win the first round and defeat Jun, the fun begins...
... and Jun transforms into Unknown, effectively ending the decade-long debate on the character's identitynote well, sort of; Unknown in the first Tag was originally meant to be Jun's younger sister. Unlike her Tag 1 incarnation, she does not mimic other cast members a la Mokujin. Instead, she retains Jun's moveset as well as gaining a damage output increase and a few boss-exclusive moves that borderline on downright unfair, including a call back to Jinpachi's infamous stun. There are no guarantees in this fight; if she wants to, she'll either initiate a simple combo that shaves off roughly half of your health or reverse your attacks to death. One of her most dangerous attacks is a Snap Back-esque move where a portal appears on the ground. If you get hit by it, a huge hand surfaces from the stage and splats you, forcing you out of the arena and leaving you with only a pixel of health remaining and a large chunk of recoverable health. Did we mention that she can combo into it? Good luck when she pulls that off. Only plus side is Unknown is not as defensive as Jun, so it's really a toss up on which side of her is worse. It depends on your play-style. More aggressive players will find Unknown to be much easier than Jun, but that's like the difficulty going from impossible down to joystick snapping.
An update for the free-to-play Tekken Revolution added a special version of True Ogre that can be fought in Arcade Mode, known as Gold Ogre. This version of True Ogre has had its AI buffed and its moveset modified to make it more boss-like. The computer will read your inputs and counter with the exact move necessary to counter it, which is usually its extremely damaging Invincible Move, now made faster than the regular version of Ogre and also safer when blocked. It can also teleport during a juggle like Azazel and appear on the other side of the screen to ram into you or breathe fire at you.
Said update also added Mokujin and its model swaps Tetsujin and Kinjin. However, these versions of Mokujin do not mimic other fighters each round like they usually do. In this game, they have all of the best moves from every character, just like how Combot can be customized in Tag 2 (they even use Combot's stance). Some of these moves have been made much cheaper, like Feng's f+2,1,2 and Bruce's knees now doing many more hits than usual. They can also do stuff like pumping out 4 Electric Wind God Fists in a row to shave off a large chunk of your life. In the Turbo Mode of Mokujin Rush, this gets even worse as they gain access to more ridiculous combos, like being able to do Paul's Demolition Man off of the aforementioned EWGF juggle.
The first game in the Soul series, Soul Edge/Blade has Soul Edge, who would later become Inferno. He is a much tougher version of Cervantes who is able to teleport and use flying unblockable attacks, seemingly from anywhere on screen. His difficulty is so great that the original arcade version of the game had to have its difficulty lowered for a rerelease, because him and Cervantes (who you face before him) were considered impossible to beat. It can still take several retries to defeat him and that's only if you're lucky, as he's utterly unpredictable. And note that "lucky" here means you happened to not be in the way when he used one of those flying attacks and he launched himself clear out of the ring.
If you did beat Cervantes and Soul Edge using Mitsurugi, you'd find yourself up against Tanegashima, a Bonus Boss who starts the fight with a matchlock rifle, fully loaded, primed, cocked and pointed at your face. You have a split second (1 or 2 frames, at most) to dive to one side or his bullet will result in an unblockable One-Hit Kill. Of course, if you manage to do that, you've got plenty of time to run up to him and hack him apart while he reloads, but occasionally he does manage to block your attacks (which somehow doesn't interfere with his reloading) long enough to get off another shot.
Soulcalibur II makes use of this trope with Inferno, a final boss who uses random move sets of other characters. This final fight carries the deceptive appearance of a single-KO battle, but Inferno's life bar is in fact 3 times normal size. He also receives a damage bonus, a special throw that knocks off more than half of your life gauge, and changes move sets when he hits 2/3rds and 1/3rd of his total life (and if you deal extra damage, it always truncates it, so Inferno starts each "round" with that amount of life). Additionally, flames blanket the stage and cover Inferno's body, making it harder and harder to discern his movements. Definitely a joystick-snapper.
Soulcalibur III utilizes this to a degree even with non-boss characters, although there are certain moves that can be used to circumvent the seemingly precogniscent AI. But the secret boss, Night Terror, is absolutely batshit insane in this regard, with Eye Beams and a stance which renders him immune to damage; unless you're using one of the aforementioned AI-killing attacks (or a few more that take advantage of Night Terror's absurd size) it appears that you can only beat him if the CPU decides that you can. Oh, yeah... you can't beat him by "ring out", either, because he'll just fly back in. Thanks God he isn't playable, at least if you're not using a cheating device.
The story mode only character Ceres is supposed to be this - he blocks pretty much every move you use against him, and the story moves on if you lose the battle. Beating him results in nothing.
Soulcalibur IV actually averts this, believe it or not, with its final boss, Algol. While he's fast and strong, he never feels impossible, and his projectiles are not only blockable but they can be parried to reflect them back against him. In the Tower of Lost Souls Mode, he actually does appear to fight like an SNK Boss, until you figure out the bug in his AI. In the right circumstances, he'll ring himself out, every time those circumstances are presented to him.
In Tower of Lost Souls mode he has a 50% chance each SECOND of "unblockability" - which includes being impossible to parry - being granted to his next attack, takes 30% normal damage, does double normal damage, has two health bars, and drains life with every attack? Essentially, if he hits you the battle is probably lost.
He also boasts some stupidly broken infinites, although the A.I. never seems to make good use of them.
The Apprentice in Arcade. While not quite SNK level, he does seem to have an uncanny knack for sidestepping verticals, vert-jabbing horizontals, air-juggling and... to cap it all off... a truly ridiculous frequency of Just Impacts.
Soulcalibur V has the honored privilege of granting us not one, not two, not three, not four, not five, not even six, but seven SNK Bosses, all fought in a row, in Legendary Souls mode. This, ladies and gentlemen, is the new Score Attack. The AI becomes demonically possessed, reads your inputs and reacts with unbelievable timing, and does not let you so much as breathe when it goes on the offensive. It can also perform any move requiring a Just Frame perfectly, resulting in combos which are completely impossible unless you have close to exact timing.
First up is Kilik. Despite being a mimic character normally, in Legendary Souls he is stuck using Xiba's style, which was originally his own from previous games. The second you let him get momentum, he starts comboing you like a maniac with ambiguous mixups, and doesn't stop.
Next is Nightmare, who uses an exclusive costume just for this mode, his Soulcalibur IV outfit. With his immense range, he will constantly knock you down and take away about 50-70% of your health bar using his Brave Edges and comboing into his Down+A+B attack while you're on the ground, resulting in a special command throw.
Your third opponent is Siegfried, who also uses his Soulcalibur IV outfit exclusive to this mode. While he doesn't have the damage output of Nightmare, he does have the momentum and mixup game of Kilik, and is also a massive block-whore. Be prepared for ring outs several times over, and good luck trying to break his defenses.
Contestant #4 is Cervantes, decked out in his Inferno costume from the very first game. While he's a bit easier than the first three opponents, your jaw is guaranteed to drop as he juggles you with Instant Geo Da Rays/Psycho Crushers for insane damage.
After Cervantes, you fight Edge Master. Just like Kilik, in this mode he gets stuck with one set style - Alpha Patroklos', who in turn uses Setsuka's style. If the AI blocks you even once you will be punished with either a combo into his Critical Edge which can be done off of pretty much anything due to how fast it comes out, or a Just Frame Sakura Twister, which is so fast it can be done twice in a row and deals amazing damage for a combo ender. For many, this is the breaking point, but there's something even worse after him.
That "something worse" is the 6th battle against Elysium. Again, she uses a set style - Pyrrha Omega's, to be exact. And this just so happens to be Sophitia's style, who Elysium takes the form of. Unfortunately, this version of Sophitia's style is faster, and about 10 times cheaper, making Elysium a total bitch to fight against. She spams Angel Strike (her signature stab) like it's going out of style, and will always end it with the Just Frame ender. She will also use her exclusive Critical Edge, which is a 20-hit full-screen attack, at the worst possible moments. Have fun.
Your last opponent is a familiar face - Algol from Soulcalibur IV, who seems to now be taking fashion advice from Gill. This time he's gotten smarter and doesn't ring himself out on accident anymore, which is bad news for you. Luckily, he's easier than the first 6 opponents put together, making this an Anti-Climax Boss.
The catch is that, while manageable compared to the other six, Algol is still a force to be reckoned with, even after losing the infinites that banned him from Tournament Play in IV. If he builds up his meter, Algol will use his Brave Edges and Critical Edge with sadistic glee, all of which puts you in position for more combo madness like rebounding you off of his levitating throne for an untechable juggle and then using his projectiles to pop you off of the ground, air controlling your position as a setup for combo finishers that allow him to further press the assault on wake-up. And if Algol is feeling especially malicious, he'll store up his meter and use all of it at once. Nothing quite says frustration like a 10+-hit, touch of death combo. Or Algol using his complete Critical Edge at full meter, which is among the most-damaging single-hit moves in the entire game. Moral of the story: never let Algol gain momentum. Never.
Fighting Layer has Vold Ignitio, the final boss. The first round is no big deal, even if he's tougher than most of the previous fights. If you win a round against him, he goes Limp and Livid, and the difficulty skyrockets. When you attack him and he's not trying to get up or in the middle of an attack, he will swap places with you and counterattack with your own move. He also has an unblockable diving attack that is guaranteed to hit if you can't get out of the way. Finally, his gun super is fast, unblockable, and heavy hitting.