In order to defeat the Dark Lord and save the world, a small-town youth must put his life in peril as he and his companions travel to the fires of Mount Doom in order to destroy the Dark Lord's ring of power.
In a world where a thousand years ago, a hero set out to destroy the ultimate evil and instead turned into a Dark Lord himself, a motley crew of thieves and con men scheme to bring down the empire he created to oppress them. (Actually I'd split this into two sentences and just say "A thousand years ago, a hero set out to destroy the ultimate evil and instead turned into a Dark Lord himself. Now a motley crew of thieves and con men scheme to bring down the empire he created to oppress them." Two sentences, but fewer words.)
Ten years ago, a city of gods among men was transformed into a city of the damned. When a prince awakens one morning to discover he has joined the walking dead, he must reverse what happened ten years ago or go mad trying.
It's just the basic idea boiled down, including what makes your story different from anyone else's. Obviously this sentence for Lord of the Rings would sound cliche today, but this is how it might have been pitched back in the day (not that it actually was).
I think these would be good quick pitches, enough to pique the editor's interest so he or she asks for more information. Then you give a couple paragraphs more, perhaps about the basic characters and/or magic involved, and if they still look interested ask if you can send them the first three chapters. You don't want to monopolize their time.
There's more than one way to pitch, but it's important to keep it quick, interesting, and to the point.