....I'm speechless, really. No thread on this?!

Anyways,

Metal Madness (Sonic Heroes) : http://www.youtube.com/watch?v=AGdyT3zNf24

Final Hazard (Sonic Adventure 2: Battle) : http://www.youtube.com/watch?v=o0ncJ5Ijftc

Mecha Bowser (Super Mario Sunshine) : http://www.youtube.com/watch?v=jX4CVnfYcd0

Bowser (Super Mario World) : http://www.youtube.com/watch?v=vctZg1TItQA

Aqua vs. Terranort (Kingdom Hearts: Birth By Sleep) : http://www.youtube.com/watch?v=HUZ6CBNUlQ0

Dark Gaia (Sonic Unleashed) : http://www.youtube.com/watch?v=22bkrsNC2Pc

That's all I have for now, post your favorite boss fights of all time!

Cloud n Candy (Yoshi's Story) http://www.youtube.com/watch?v=FV-XTe7z-q8

VS Cortez, Paper Mario TTYD: http://www.youtube.com/watch?v=jqeQZoA8tfg
VS Magnus von Grapple V.2.0, Paper Mario TTYD: http://www.youtube.com/watch?v=fKCPOrfXdec
VS Masked Dedede, Kirby Superstar Ultra
VS Gruntilda, Banjo Kazooie
yup. Gotta love the Music.

Ultima Underworld I: Fight against Tyball http://www.youtube.com/watch?v=NoqsrFP9rpE
Excuse the foreignness of the video. I love this battle, it's really challenging (Unless, of course, you have a scroll of Freeze Time )

Marx Soul from Kirby Super Star Ultra (Final boss fight in The True Arena). Seriously, he's so hard to beat, I got KO'd by him five seconds after the battle began. And he looks so twisted and savage...He's weird but awsome. XP

I like Giygas and the fianl boss of Mother 3 but it made me cry, its so sad

Yourself x2 - Ninja Gaiden Sigma


Vigoor Emperor - Ninja Gaiden Black

GLaDOS - Portal


And many more i might have forgotten :/