And the last 3 patches zerg has gotten better, more or less.
So why aren't zerg the best?
I think that by realizing that the game will balance itself out, blizzard is being too passive. You could give zergling +1 attack, roach speed bonus, hydra much faster, and still you would have some sort of balance because strategies will evolve.
Only it wouldn't have to be zerg doing it.
My main point here, despite my obvious bias, is that balance isn't the same as letting people figuring out strategies depending on how much they need to. I think that blizzard has to change a lot more and not be so careful, because honestly it will just lead to another race having to come up with a smart strategy instead of z.