A curious game. The only way to win is not to play.
Yes, you will draw against the double rack deck. But I think that if we find a deck that can, at worst, draw against all the others, that’s the best possible. The rack deck is not this deck, because it loses to the goblin vandal. But if we did find one, then it wouldn’t matter if every deck could draw against it, because it would never lose, and would beat some decks. Such a deck would be the best possible deck.
The closest thing that I can think of to a big blue creature like that would be Chronatog, which is U1 for a 1/2, that can get +3/+3 if you skip a turn. It’s only a 4/5 every other turn, but you only take damage from the racks on your turn, which you will be skipping half the time, so it’s just as good.
However, my understanding of the challenge makes it so that FoW, Blue card, x, is not a reasonable deck unless the x and the blue card are named. There has to be one particular configuration that will beat (or draw) all comers. Saying that “if your deck is this, then my x is this” doesn’t mean that that is the optimal deck, only that the suggested deck is not the best, as it would be beat by Fow, blue card, x.
Oh, and city of traitors, sol ring, phyrexian processor would beat the rootwalla deck (or, really, any creature based deck) into the ground in a painful way. But would of course die to FoW, blue, x.
Also, how are we to deal with random effects? It seems like we have to be able to deal with the worst case, or is it enough to solve for average case? If a certain deck won against all others with a >50% chance, then that would be the best deck.