The Doomsday Machine from the Star Trek episode of the same name.
I’ve always liked one they used in (I think) the Skylark series.
Use a matter transmitter to simultaniously move every star from galaxy A to close proximinity to a star in galaxy B.
At the same time, move every inhabited planet from galaxies A and B (except for our target) to a compatible orbit around some star in galaxy C.
Every star in galaxy B (along with every star moved in from galaxy A) goes supernova at the same time.
Instant quasar. Ouch.
While I think Ice-Nine is a good choice (I happen to like it because Robert Hunter and Jerry Garcia named their music publishing company “Ice-Nine Music”), my vote, too, goes to the device in Dr. Strangelove - “You can’t fight in here - this is the War Room!”
And I’m chaning my name to Buck Turgidson.
That would be the “Doomsday Device”.
“Why would you create a doomsday device and NOT TELL ANYONE?!”
Personally I would create a world destroying evil and then entomb it with a seal that says, in ancient cueniform, “Ancient Gold of Hammiraba,” and is a pun in even ancienter cueniform, “Ancient world-raping death monkeys,” so that the sidekick/translator can say, “Wait, I think there’s another… AAAAHHHH!”.
:), Cue ancient tape recording of “BWAHAHA… end tape”
Why, with that plan to reveal, I wouldn’t have to pick up a check at the Arch-Villan cafeteria for weeks!
I’ll always have a place in my heart for the “Unbeatable Strain of the highly contageous virus”. Ah… the beauty of it…
A Huge Comet. Simple, yet effective. Plus, we’ll see it coming months in advance, but won’t be able to do anything about it. I’d like to see what people do during their last months on earth.
For immediate destruction of the planet, you simply can’t beat the device I’m building in my gara-- err, never mind…
A Giant “Lazer” put on the moon to blow up various cities unless they pay me 100 Billion dollars!
Kirk’s corbomite device. I’m still trying to dope out the schematics.
Little black holes, used in several science fiction novels and stories (sometimes intentionally, sometimes accidentally). A good one if you like to have a period of time when you know you are doomed and can’t do anything about it, as a small black hole could bob around inside the Earth sucking up matter for some time before there were any noticable changes on the surface.