Your favorite boss battle.

Two of them.

http://www.youtube.com/watch?v=YCWoTa5GlWg - Devil May Cry 3. Third Vergil. Nothing else.

http://www.youtube.com/watch?v=C4G9288sa3Q - Devil May Cry 4. Savior Battle. Nothing else.

Andrew Ryan in Bioshock.

Sephiroth No doubt about it.

The Devil in Sheol, from The Binding of Isaac. The music is epic, and you're stressed as hell when you get there the first couple of times (and every time an achievement is in the balance...)

The Operator in The Witcher 2!

Fight vs Twin Blades in Fable: TLC

Vergil from Devil May Cry 3 of course. (All of his boss fights, I just picked this one because the video is awesome. :D )
Also (Spoiler... kinda) Credo from Devil May Cry 4.

Let's just say all DMC 3 and 4 bosses and keep it at that. :D

This

I dunno, I like bosses from PoP Warrior Within, DS 1 and 2, DmC and games like that.

Can't remember much boss battles, but the one with Dante vs Geryon the Timesteed was epic and beautifully executed

i don't have any favorite from a singpleplayer - but when i played WoW Yogg-Saron was probably my favorite bossfight - great challenging mechanics, even thought it was in the easy-peasy WotLK - but ofc i loved bosses like Nefarian, Onyxia and Ragnaros in vanilla wow

who can forget this

this was awesome :P

This is easy, best boss battle ever is Psycho Mantis on the PS1. That was The Shit!

Kain vs Raziel

Raziel vs Kain

Legacy of Kain: Defiance

Dante vs Vergil; final battle
Devil May Cry 3
EPIC BATTLE
(provably not the best video)

I'd say all from The Legend of Zelda: Ocarina of Time but the Bongo Bongo one.

Planescape:Torment when you have a certain blade.

I have to plead a soft spot for Deathwing, but oddly enough also Ultraxion. I was usually the healer who didn't muck it up because I knew how to hit the button!

So many great bosses over the years. The end bosses of MGS3, Portal 1/2 and FFVII were perfect for those games. Dark Souls has so many notable ones (Sif & Artorias get a special mention). MGS had some great ones too (Psycho Mantis and Sniper Wolf).
My favourite has to be Kalameet from Dark Souls though. He gave me such a huge pounding, it took me quite a while to beat him and cut his tail in the same fight.

P.S. I also want to note here my least favourite ever "boss" battle... the end of RAGE was truly awful.

Any fight with this track playing.

france in europa universalis 3

• vs Sephiroth (FFVII)
• vs first Tigrex (Monster Hunter Freedom Unite)
• vs first Dragon encounter (TES V - Skyrim)
• vs Captain Flynt (Borderlands 2)
• vs 'insert any "The Legend of Zelda" boss battle here'

I always loved this one, though it's really short, I tried to not use any reaction commands to make it last longer.

Also, I love this one due to its music, I was amazed when I fought him for the first time seeing as he clapped at the rythm of the music. Also I didn't know about the sponge, I hated the fact that the music changes when you use it. I can't find a video where the guy doesn't play too fast to enjoy the tune, so I'll just live the theme if someone wants to hear it.

I don't think it needs explanation when you look at it.
Not the best, but man my breathing stopped for a second at the sight.

Best would be maybe Belial from DMC4. Prince of Persia games had awesome bossfights too.