I prefer a format with as few dominant decks as possible, like Adam. It’s much easier to create a deck meant to beat the format when there’s only a couple decks to defeat. When there’s a bunch of decks out there that can take 1st place on any given day, you can only cover so much with a “silver bullet”.
A silver bullet is a deck meant to beat everything. I consider our Queendom deck of Worlds 2005 a silver bullet. We worked on getting it able to beat any deck that we may have come across at Worlds. Given that there weren’t many archetypes, it was a little easier. It helped that Grass was a nonexistant type, Nidoqueen’s weakness.
If you can’t beat ’em, join ’em. Can’t find a silver bullet? Then use whatever the best deck is at the time, but make sure to use a tech to give you an advantage in the mirror match. Last year, I preferred Blastoise from Crystal Guardians in my Gardevoir/Gallade deck. It prevented weakness on all your Pokémon, so an opponent’s Gardevoir could only do up to 60 damage to your own Gardevoir or Gallade, giving you the chance to OHKO theirs while they need 2 shots to take out yours.
(This post is a response to this poll. Be sure to vote and take part in the discussion if you have not already.)