well in our starting booster, we have 2 stack containing 2 nymphs, which are, despite their power level, pretty much unpickable stacks...
Compare this with an Antimatter-Reverse time or a Graboid-GoTP stack.
Or with the Phoenix(most players would say it's the best card in the game)- Ice shield (usually the main reason to play water) one.
Or with the voodoo doll-freeze stack, which is a half deck in itself.
3 levels would be okay, in that case the "average" and "powerful" categories should be merged since most of the cards are "average" or "powerful", with a few "very powerful" and "poor" among them. It would be impossible to balance stacks of 3 by placing one card of each power level into them.
Generally, most stacks could contain "average" cards, while the ones with a "very powerful" would contain a "poor" as well.
From the top off my head I can name only a few which can be considered "very powerful": Phoenix, Graboid, then maybe Nova and Discord. "poors" are equally small in number.
But of course this would need a more throughful evaluation.