just so people know what we're talking about:

This is NOT the most powerful card in magic, and never will be. It does NOTHING on it's own.
Secondly, Black Lotus can also not be considered the most powerful card, it ALSO does nothing on it's own.
In fact, the entire discussion of what is the most powerful is pointless rhetoric, you need cards to work together in order to have an effect, and thats why this game is great in the first place, because of the nearly infinite varieties you can generate. look at Flash Hulk. for a brief little while it was the most devastating thing anyone had done in ages, now it's just one of any dozen "combo" decks.
Please, remember what this thread is about, what is Best, not the most powerful.
My answer is that the lotus is NOT the best card, Opening hand it's a great thing to see, and can set you on a pace that ideally overruns your opponent, but the rest of the deck needs to keep up.
I will say that the lotus combines the best of power, playability and RARITY though, and THAT is why it is worth so much.