Gardevoir will win worlds for the same reason why Darkrai won worlds in 2013. It has good match-ups into everything, and the one bad match-up is winnable. You have to look at this from a numbers perspective.
Majority Decks:
Gardevoir
Garbodor
Ninetales (with/without Owl)
Metagross GX
Underdog Decks:
Fire
Darkrai
Golisopod
M-Ray
Zoroark BREAK
Greninja BREAK
Gardevoir has a winning match-up into every one of the decks listed save Metagross GX and maybe Greninja BREAK (not to familiar with that matchup).
Now you know a majority of people will play Garbodor after it won Internationals, and you know people are FOTM Ninietails becuase of its success at Internationals. Not to mention the Gardevoir Hype. So i'd say about 66% of the worlds decks are those three decks. Metagross GX, being the counter to the three, will be around 14% of the worlds decks. The last 20% being the underdog decks. (Mind you this is all speculation) So if Gardevoir wins the matchups against everything expect say 15.2% of the decks there, that means that 84.8% of the games Gardevoir plays, it should win. So unless the amount of Metagross players jumps up drastically, Gardevoir has the best chance of winning worlds.
In order of how the decks will place
Gardevoir GX
Garbodor
Metagross
Ninetails
Zoroark Break
Fire
Darkrai
Greninja BREAK
M-Ray
Golisopod.
(Maybe some changes below Ninetales, but the top four are locked)