Who has been the boss fight that was the hardest for you?

If your boss does contain spoilers for something I would recommend spoiler tags, But anyways lets do this..

My Hardest boss that I've faced is Undyne the Undying.

Because of three reasons
1. [you must login to view links] <-Curtain Number one
2. Some of her attacks freeze you in place just as her attack turn ends and if you get hit, OH WELL! You still take an extra hit of damage.
3. her damage outweighs the ammount you can heal on one turn.

My god, the hardest boss ever? Probably the dragon thing from Hi Hi Puffy AmiYumi, usually you can just dodge attacks by flying the bus above the bosses, but the power ball things follow you, not to mention you can only attack him if you shoot at his head.

That sounds tough just reading about it `o-o

Yeah, and I only had one bar of health. And the only way I beat him was using slow motion, and all 3 jam sessions

Oh, well with Undyne it was PURE. LUCK. XD

For me, I'd say Marx Soul, Giygas (still haven't beaten him), and the Li-grim from Final Fantasy Tactics Advance.

(Hey Ryu, long time no see.)
i heard Gygas was tough, I guess I'll see for myself when I get to him. :3

I actually, I heard Giygas himself is quite easy. It's the debuffs leftover from the battle before are what makes it harder.

The hardest I ever faced has to be Red from Pokemon Heart Gold/Soul Silver. I barely beat him after I decided to teach my Meganium Earthquake and had my Mewtwo join in for the fun.

(And then a wild Sans appeared!)

To be honest, Red wasn't that difficult to me (Okay I had a Lvl. 100 Arceus, and other Legendaries but still)

I wasn't as good as a player as I am now, but I guess I could take him down again.

Though after posting this, another boss came to mind...

Trashbot from Mario & Luigi: Bowser Inside Story.

I remember very little (it was probably that hard--I must've wanted to forget it), but I do recall that at random, Trashbot will literally shove Luigi down a mini-Trashbot, and make you play that rearranging-cups-and-you-have-to-find-the-object thing. And if Mario gets beaten before his brother is freed, too bad! It's game-bleepin'-over! It was even harder than the also-difficult final boss, Dark Bowser! (And that's saying a lot.

Wow.

Then there's Master Core on Intensity 9.

You have 2 lives to beat Master hand/Crazy Hand
Master Core Giant
Master Core Scorpion-Cat thing
Master Core Swords
Master Core Clone

and then Master Fortress
I've only done it once because they relentlessly attack you with no cooldown(Except when their in the middle of an attack)

Mecha Sonic from Sonic the Hedgehog 2.
God, I STILL haven't beaten that dumb tin can. DX
(And no, Mecha Sonic and Metal Sonic are different robots. Complicated, I know.)
It's on the Death Egg Zone, and there are no Rings on that stage. Mecha Sonic's attacks are so freaking unpredictable, and by the time you figure out what he's up to, it's too late to dodge. And since you have no Rings, you die upon impact. (Heh, guess I know WHY it's called the DEATH Egg Zone now...) And then on top of that, when you CAN hit him, you have to be careful on WHERE you hit him. If you touch his spines, you die. If you touch his front, you die. If you touch his back, you die. It has to be PERFECTLY on the top of his head. So gosh dangit that bossfight is so hard. DX
And as I mentioned before, I still have not beaten Mecha Sonic to this day.

So far the Fume Knight in Dark Souls 2: SOTFS, still haven't beaten it