Ok, so, you're evil and going to kill everything, we get that

[slightly evil]

the plain and simple enter key, black and shiny on this particular keyboard. It doesn't need to be flashy and it's functional but at the right moment, right before I activate the magnetic disruption ray, overriding the magnetic forces that hold all atoms together and turning the world to dust; It'll feel apropriate.
(I know this was the reality bomb on Dr Who, but I honestly thought it up before them)

 

[Wing Dairu]

Mine is a bright red button mounted on the wall with "THE BUTTON" printed on it in big, bold white letters. It's just the right size to slam with my fist as I back away from the hero, and it causes every hat in the world (except for my own hats) to turn into...
THE CHEATER'S LAMENT.
Oh, I'm so bad...

 

[Phantomess]

A very large, red button... suspiciously like the one in "The Christmas Invasion" that David Tennant hit with the flat of his hand.

The one that looks like a game-show buzzer.

Yeah. That one.

It will... it will instantly... I dunno. Give me freshly cooked pancakes?

 

[Ninonybox_v1legacy]

a fist sized blood red button with a glass caseing with a countdown hologram above it labeled OWNED NOOB

 

[espilcEkraD]

My Buttons big and Red with the title "My F*** You Button" and when pushed streamers and balloons fall from the roof and a banner unravels that has "F*** You!" Written on in and then the world blows up 10 seconds later.

 

[The King of Rock and Roll]

I don't need one seeing as I already rule the world but nobody knows it.

 

[Angerwing]

I'd have it on a pedestal at the end of this huge obstacle course, requiring, speed, strength, and exceptional puzzle solving abilities. It would be a trying test that only the greatest of heroes could finish, located deep within my heavily armoured base. There would be an eye scanner, fingerprint scanner, DNA tester and weight tester that leads to my special path though, so I don't have to make it through. This button would be red, and have the image of a grey skull on it. Next to it would be a light blue disarm button.

Both buttons will shut down the obstacle course, and pour acid down on whoever is in there, shown to me via CCTV for my amusement.

The real button will be on my wristwatch that will require a 10 digit PIN.

 

[SinisterGehe]

mine is the size of a doorbell button and is installed next to my actual doorbell if you push the wrong one you will end the world not me and i will laugh at you

What If i Knock?

 

[LandoCristo]

It will be a fuse, but instead of lighting it, you pull on it. This will then cause party balloons full full of explosives to rain down throughout my base from the ceiling, and then I run away before they all blow up.

 

[Lucifron]

My button is covered by bullet-proof glass in a yellow-and-black-striped stainless steel frame. The button itself is as red as blood, and it has a yellow text which says "Going, Going, Gone Fishin'."
The only way to activate it is to strike it with a massive wooden mallet, on whose striking surface there is a simple message to the world:

"How about a nice cup of Shut the Fuck Up?"

Above the button a there's a sign which says: "Break glass to ensue hilarity."
Who knows what the ensuing hilarity is? Perhaps God, but certainly not You.

 

[TyranidXX]

I'd have two; your simple, generic red button on a console somewhere that will make pancakes of the best quality and... a light-switch.

that's right, a light-switch... my office will be illuminated by a lamp on my desk and if anyone used the light-switch... not even I know the outcome.

 

[RatRace123]

My button is big and red and shiny, and it says DO NOT PRESS on it.

So damn tantalizing.

 

[Serenegoose]

Because I have an ounce of sense, and have read The Evil Overlord List, the button would be marked "sewage overflow containment" and would be in a small, nondescript room in the maintenance section. The button next to my throne would be grand, red, require 2 keys (neither of which are on my person, ensuring the hero does not feel a need to kill me first) and it would trigger a hail of bullets on the person that pressed it.

 

[Miumaru]

I dunno, but the head of a cane flipping open with it inside seems cool.

 

[Eternalsun]

A huge red button in times square! With the words do not press on it. People wont be able to help themselves. and they will destroy themselves with millions of Flying robot worms that are indestructable and have lasers and can go as fast as a bullet.

 

[Sir Thomas Sean Connery]

My button looks like that drunk gay guy eying you from across the bar. When pressed, it immediately turns the entire human race gay, thus causing extinction.

 

[Malkavian]

My switch is under the top of my cane,, right next to the button that activates the spring knife in the tip, for easy accesability.

 

[Billska]

I'm gonna press the awesome button.

 

[cthur]

My button is yellow and has a big ^.^ face on it
It is pre-pushed and held down by a glassful of grape juice.
When I am done ranting about the cruelty of the world (without giving away details of my plot), I will say, "I'm parched" and reach for my glass of juice, making the button decompress

KABOOM!

the world has kerploded