I'd have to say that it depends on the Pokémon. What are their Natures, EVs, IVs, Stats, and Movesets? If you don't know what they actually mean, do some research. Their Natures determine the general growth of their Stats; decide whether or not they have a beneficial or hindering Nature depending on their Moves, or change their moves according to what would work best with the Nature they have. Investigate their EVs and IVs and then decide on your own from there.
It may not actually prove helpful with this, but your own opinion could be worth something. Besides, it's all what you make them to be. If you decide you want White Kyurem to be an overpowered beast, and Zekrom to be a mediocre Pokémon, then you can make them like that.
Honestly? With Pokémon, you can do almost anything and mold everything to your own unique and refined image.
If you really can't decide, then switch them out routinely. That's what I do with my Pokémon party. It helps a lot, because then I won't have to rely on just six Pokémon if things go wrong along the way. I rotate my party depending on what would benefit me at the moment.
This may not help, but I sincerely hope it gave you something to think about.