I agree on the timelessness of potential Power Nine Pokemon TCG cards.
I guess another potential prerequisite would be, for lack of a better term, if the card is a perfect example of an abuse of power from within the game, to the degree where players would ask "what were they THINKING when they made this card?!"
Oh, and it would have to cover cards spanning from all the way from the beginning; none of this DP-on or whatnot. And, again, the card would need to be something that is powerful today as it was when it was first released, possibly even stronger when combined with new cards that weren't considered by the time it was initially replaced; essentially its power has NEVER diminished since its release.
There are plenty of good suggestions here, many of which I didn't even consider initially (such as NG Cleffa). When I get a chance I'll compile them into a single list and give my reasons for why something was or wasn't added to the list, and maybe someone could agree or disagree with me.
I also figure that, if there are more than nine cards worthy of being known as the "Power Nine", I would either choose the nine oldest cards, or maybe revise the whole deal and call it the "Power Twelve" or whatever the final number is. In any case, the final list should contain a limited number of cards of unquestionable power as well as showcasing the supreme, well, LAPSE in R&D's better judgement.