What is your favorite boss fight in a video game?

Despite how frustrating it is... and the fact it took me about a week of non-stop trying.
Kingdom Hearts Birth by Sleep's Mysterious Stranger.
I was so proud of myself when I beat him on hard mode.

I love intrusion 2 bosses, the last one probably being the best

Terraria: the moon lord. I don't think alot of people will agree with me but when I first got Terraria it didn't even have that boss in the game yet. I used to play Terraria when it got released and I had so much fun. Then I saw that it was on steam and that it got updates so I checked it out and loved it. I like alot of the bosses actually but defeating the moon lord made it feel like I kinda completed Terraria

Kefka, FFVI, for the music alone.

Mr. Freeze, Batman Arkham City, otherwise. I liked having to avoid him and figure out what to do to fight him.

Sif in Dark Souls :)

I actually can't think of a single one.. I can name my favorite anything but bosses? That was hard :P

all bosses in this perfect game

View attached image.

C'thun, Thaddius and 4 Horsemen in World of Warcraft. WoW actually has a lot more amazing boss fights but these would be my top 3 picks.

Hyperblast, vs Xan (UT99)

It's soooo hard to pick just one...
I guess I'll go with Vergil from Devil May Cry 3, but I can think of a lot more bosses that are just as awesome (quite a few of them from the same game XD ).

Top 5 Best:

1. Lady Maria of the Astral Clocktower
2. Sister Friede
3. Artorias the Abysswalker
4. Abyss Watchers
5. Burnt Ivory King

Top 5 Worst:

1. Defiled Watchdog of the Old Lords
2. Loran Darkbeast
3. Darkeater Midir
4. Orphan of Kos
5. Nameless King

Malkorok as a spriest was fun as hell.

DS3 - Soul of Cinder

World of Warcraft back then : Mimiron - Hard mode (10 men)

Loved the difficulty and chaos with this boss.

The Boss

That demon thingy in Far Cry 3.

Ornstein and Smough. Best duo ever

The dragon from dragon's dogma.
Not really because it's the best fight, but because I stumbled into it while playing with pretty much the worst gear in the game and an under-leveled class, so it took some thinking to figure out how not to die with the shitty set up I showed up with.

I love boss fights in Dead Rising games or specificaly, those in 1 and 2, hated the Off the Record ones because they nerfed their health so baddly, and the new last one....fuck that boss..., other than that...i dont know or remember whats my favourite....oh, i guess this one? = https://www.youtube.com/watch?v=fmu52vB6hWo god i love this game....hate Sony for shutting down the studio, saying the game wasnt good...when infact it was...but wasnt advertised at all aside from trailers as far as i know...of which ones i never knew of....
Oh yeah! I love the bosses from Adventures of Batman and Robin too, love em = https://www.youtube.com/watch?v=VpmTTTohgRc
Oh my god, HOW could i forget the Jurrasic Park bosses goddamnit like the last Rampage Edition one = https://www.youtube.com/watch?v=j4bPVvCi3sY do note that i had NO IDEA the grenades would make it easier, when infact i struggled like hell with the other weapons...anyway, as a little kid i as both scared shitless and amazed, like, look how the graphics still look amazing, esspecialy the water, just incredible.
Okay, i may not love the actualy boss fights as much, but i LOOOOOOOOVE the boss fight musics in these games <3 = https://www.youtube.com/watch?v=uXnEh-poxdk
I actualy loved the fairly unique looking bosses in Ghostbusters = https://www.youtube.com/watch?v=Z-09EkfnMmY also again, love the music so, so , so much.
This games bosses and their music was sick, and the sound effects, god i love em, Konami was so good back in the day. = https://www.youtube.com/watch?v=-7kSYLCOYgo

Pretty much every battle in Shadow of the Colossus, but especially Hydrus (the water one) and Avion (the first flying one).

Dark Souls - Ornstein & Smaug, Artorias, The Pursuer
Hyper Light Drifter - The Hanged Man, The Hierophant, Judgement
Furi - all of them

MGS3's The End is my favourite, the first time I fought him it took a whole afternoon of hide-and-seek sniping to finally defeat him and I enjoyed every second of it.

Ringed City's final boss was amazing, particularly because of the huge arena and the mobility needed to win. I also loved Soul of Cinder (it was like a best-of of all my DS2 PVP opponents) and Artorias (it was a huge pain to defeat him and sadly I can't seem to do it again).