Yep, there's a least favorite boss, as in it's one you struggle with, but for this thread, I want you to tell me which one just....annoys you so bad you just don't want to get to that point in the game. That one boss, whether Sega, Nintendo, or Sony...Perhaps the xbox? That really just grinds your gears! Mine personally is the Great Mighty Poo. Oh yeah the first time you run into it it's all laughs. But ten years later it's annoying, overdone, I seen it a million times on youtube, in game, and well, it's just old hat to the point of where it's discusting! Yeah, most of you will think, "WHOA manic, I love that boss!" But there you are~

manicman66 : Probably the Labyrinth zone boss from Sonic 1. I know I have seen people easily complete it before but I for the life of me cannot stand that battle against Robotnik. Not to mention you are trying to avoid drowning from the water, dodging spikes and Robotniks attacks all at the same time. Grr I hate that thing! XD

I agree with Solidx, but, the most annoying boss I have yet to beat and even completely gave up on is... the Gradius and Boswer battle. I'm counting it as one battle for the sake of the thread, I mean, after the Gradius fight, I'm all out of juice, and with the Boswer fight straight afterwards.... R.I.P. Caring2Win

Once, I really hated one boss from PacMan World 2 on the GameCube. The Red ghost (Blinky?) controlled a flying robot that consistently shot 1-3 fireballs in your direction, and ironically I learned it's made to aim for a moving target so if you stand in the same spot you have a better chance of avoiding its attacks. And wherever the fireballs hit, it would leave fire you'd also have to avoid.

Plus the whole stage was above a lava-filled abyss. And whenever he fired 3 fireballs at once, the gamecube would really lag and the game became somewhat slower too so yeah.

I really don't have one but the one that was the most pain in the butt was the second battle with the B.O.X. Robot in Metroid Fusion it is so hard for me after that The Nightmare seemed like child's play SO FUDGING HARD!!!!!!

Most annoying boss in gaming history has to be the Yellow Devil because the first time you fight him, his pattern is very unpredictable and you have to memorize how he moves, but you probably will get a game over before you have the opportunity to learn his pattern. He is easy though when you use the pause trick with the Elec Beam.

Riku Replica in Chain of Memories. Most. annoying. boss. Ever. Not that hard to beat (if the stupid game doesn't freeze) but he is SO ANNOYING TO TRY TO GET PAST. And his one-liners are AWFUL. *sigh*

I got an annoying boss in my mind when I saw this thread.

Boost Guardian from Metroid Prime 2: Echoes.

Dark Torvis Bog's atmosphere constantly whittling away your health while Boost Guardian is being annoying.

Mashtooth from The Legendary Starfy.
The actual fight isn't the annoying part, but after hitting his second form around he shoots a giant rock (about 8.5 Starfys in height and width) at you.
You have to push it back twice by tapping the buttons fast. I repeated that fight so many times because I couldn't push it back the second time.

 Parace L'sia from Arcana Hearts 3 is one of the cheapest bosses you could ever fight in a fighting game. Hate the way she uses her elemental orbs to attack you over and over and most of the time at the end of her combo, she would do one of her specials to finish the job. Very annoying, I was never able to figure out her pattern.
https://youtu.be/DSgw9RKpaKY

I say Gill from Street Fighter 3.  He's mad powerful with his strikes and his Super Arts can take away almost all of your health.  The worst thing about him is his "Resurrection" which allows his health bar to come back and you have to KO him twice.  Probably the most difficult boss in all of Street Fighter. 

If it's a nomination for THE hardest boss, then it's Robotnik's final form in Sonic the Hedgehog 2. I mean seriously: no rings at all? At least in Sonic the Hedgehog 1 there was an easy way to avoid being crushed by the pistons and the electric balls could be avoided easily. But that machine...?

I would have to say the most frustrating Boss battle was Malpercio from Baten Kaitos: Eternal Wings and the Lost Ocean. The battle system already takes some getting used to and does rely a little bit on luck. It is a turn based battle RPG, only it is based on cards that you collect. It is like Kingdom Hearts Chain of Memories on crack. You collect many different cards, each having their own weaknesses and strengths, as well as numbers. You have to try to make combinations within a few seconds that are not only as long as you possibly can, but utilize the right strengths against opponents weaknesses. The cards are elemental, which means that you can also further the strategy by playing specific types of cards in certain orders to combine them to make new effects that can be useful. All those aspects along with different number combinations giving certain benefits and drawbacks. As you might be agreeing with, the battle system is super complex as it is. But that isn't what makes Malpercio so frustrating.
He is the second to last boss (really the main enemy of the game. The last boss is kind of a random afterthought). First, he has an INSANE amount of life, and he can revive massive amounts of HP seemingly at will. If I have an attack round that doesn't bode well for me, I can expect Melpercio to revive more HP than I dealt in that round. But here is the real kicker. Usually, turn based RPG's are fair in that even enemies don't typically get to break the rules as far as taking turns and following the order. Sometimes, there are stats or abilities that might give them an extra hit in before your turn, but it is done pretty fair. This one breaks all rules. Very rarely does he get one turn before you get your turn. Usually, he gets at LEAST 2 turns before you get to do anything. It is super common for him to get 3 turns in a row. There was one time where I counted a total of 5 turns before any of my characters got to do anything. How many times he gets in a row is random (if there is a system, there is no indication as to what it is). His attacks usually do a lot of damage. And the worst part is that almost ALL of his attacks hit all of your characters in battle at once. His attacks are so strong that 2 of my 3 characters die after 2 hits without being revived. 3 if they are lucky in which attacks he uses. I had to stick one super high defense build character in the party just to be used as a healer to revive my other 2. Now typically, the answer is to just spend time leveling up your characters. That is what RPG's usually make you do. The problem with this game is that Melpercio levels up as your characters level up. So the stronger you make your party, the higher Melpercio's stats are too. The ratio in your favor, but only SLIGHTLY. The difference in stat increase you get compared to Melpercio with each level up is tiny, and not even really noticeable.

He is just a very unfair boss. And the fact that so much of the fighting in general includes a bit of luck, and there seems to be no system to when Melpercio gets extra turn advantages, it rates as the most frustrating for me.

Silver The Hedgehog from Sonic The Hedgehog 2006 also an Extremely Broken Boss as well https://www.youtube.com/watch?v=_5njB_3qTyw.

   There was one boss in Mario & Luigi Bowser's Inside Story that was always a huge pain to fight through. That was the giant Bowser battle with the Fawful Express. It wasn't a relatively hard boss in the sense of a challenge, and it wasn't even a boring battle per say, but the one problem with it was the way to control the battle. In the other battles, you would use both your punch attack and fire attack together, and while you still use the punching in this one, the fire is the much more prevalent attack, being the only way to attack the actual train. But to use it, you blow into the mic, which can be extremely finicky at times. I had to replay this battle three times at one point because where I needed just one more good attack, for whatever reason, I couldn't get a full powered hit, and ended up losing on the last turn of the battle, which has a limited amount of turns. It was definitely not hard for its AI or anything, which made it very annoying to fight.

superdan : Damn, I guess there ARE more annoying bosses than Robotnik. It's as if Edea were able to use her powers to lift you up the ground as she does to Riona!

I've fought a lot of annoying bosses before but its hard to choose from one but some of the first bosses that come up in my mind are:

The Bed Of Chaos from dark souls because it has a large attack range it can knock you down and if you don't fall into a hole you get up only to get hit again and when you do die it takes a bit of time just to get back to the boss fight

The Trumpeter from Nocturne because of his annoying instant death attack that cant be blocked and how if none of your team mates are low on health he will re heal himself all the way

Sephiroth from Kingdom Hearts he wasn't so hard in FF 7 but then hes the superboss of Kingdom Hearts and hes really really hard my hard time is just trying to hit him at all but dat One Winged Angel :/ 

My lease favorite boss is Bowser from Super Mario 3D Land. You have to kill him twice, and avoid death. If you lose most of your lives, then you have to leave and restart. He's bullcrap, and it's no fun.

Yunimori : Im assuming you mean the Riku Replica fight 3 which i guess this could go for any of them assuming you are a high enough in levels but all you need to do is make a deck that consists of Fatal Frame, Sonic Blade ad Ars Arcanum, maybe even Ragnarok or you can just make high level card sleights deck