Or what is the MOST MEMORABLE Boss you ever face?

ngl, Undyne made me rage quit one or twice, but only because up to that point in Undertale, the fights were relatively chill

Probably not the hardest, but my immediate thought was Sigma from Mega Man X coupled with all other obstacles.

The giant frog in "Hearts of Stone", DLC from "The Witcher 3" (in the original version).  I tried literally everything, but it always ended with me face-first in the dirt. 

Obviously gonna plug these:

Sans... I still have ptsd

Hmmm.... the oddest boss would be slime boss from Secret of Mana which gets bigger and bigger as it takes more damage. So big that my team couldn't barely move at all with the whole room being taken by the slime boss.

When it comes to most memorable boss, there are bosses memorable because of bad things but there are ones of good things. My vote goes to Seven Force from Gunstar Heroes and Gunstar Super Heroes. Fighting boss with multiple forms each with its own attack pattern is memorable and challenging.

Ender Dragon in Minecraft

Mh... hard to tell, there might be much more than what i remember, so i'm gonna mention among the latest ones:

FFXII Yazmat, fighting for 3 days... it happened

FFXII the zodiac age - the 5 judges at the end of the 100 stages mode, i aided my bro in fighting them by providing suggestions on strategy, he was underleveled but they were tremendous in any case. I loved how the battle was designed for using any possible feature and trick of the game, included spacing and distances and timing... it pretty much included all i wanted the game could ever implement all around the game (i think that modding could make wonders here)

Disgaea D2 Baal (the 400 million HP version for my own mental sanity), it was so cool, never saw such a great display of features and awesomeness all in one go in a single bossfight of a strategy JRPG game, i literally had to use everything here a fight for a whole night, trying to not make any serious mistake

Rabi Ribi - Irisu (hard mode) the game is incredible on its own, this battle is quite intense, and yet i still have to fight the power-up version of it which was available only after the ps4 latest platinum edition update

Rabi Ribi - No 0% item run, almost literally any bossfight... i really say what i mean: (well at least starting from hell difficulty or it would probably become much easier) every battle will make you sweat blood, you will never forget, it will stay impressed in your skin forever. In particuar i'm doing the Alternative Mode in which basically the avg level of bosses skyrockets, compared to standard mode... i can only say that such challenge is not supposed to be handled by humans, i'm trying my best and at least 2 years have passed since i started, but i'm quite close to its end. I can say with serenity that this will be hardest challenge i'll ever do for the rest of my life.

The Sa-Matra from Star Control II

Well, likely Fantam from Breath of Fire IV.
Not only it looks ridiculous, but the boss is not even requiring any effort at all. Just make sure your entire party as full HP when you start this battle, and put the game in autoattack mode (all characters will then only do normal attacks until you break it off or until the battle is over) and make yourself some coffee. By the time you're drinking it, the battle has likely  already been won.

The deity from Hyper Demon!

The boss fight itself only lasts a few seconds, but the anticipation and seeing it for the first time has been one of the most memorable gaming experiences in a while.

ISHIN. FROM SEKIRO.

ABSOLUTE HELL.

Majora's Mask Great Bay Temple boss.

Vissiluth (Magic Carpet 2)

Most memorable boss was the big reaper conglomerate skeleton dude thing at the end of Mass Effect 2. Jump scared so hard my headphones ended up against a wall.