The power of a card is rather subjective. Thought Lash can easily remove your entire library from the game, which can be considered powerful, but without a way to harness that power...
Demonic Tutor contains the same card-getting power as Peak. Both get you one card. Peak does it for less mana and lets you see your opponents hand. But Demonic Tutor let's you pick which card, something very important. There are different kinds of power. Balance can wipe out all lands, cards in hands, and creatures for very cheap. I definitely call that power. Snowfall can produce some extra mana provided you have snow-covered islands, but the boost in mana is almost useless because of how conditional it is (really, the card can help pay for itself and only temporarily). Now, Tolarian Academy can produce plenty of highly usable mana. Some people consider that the most powerful card. How does it measure up to Necropotence? One gives you mana provided you are able to get artifacts out, the other trades life for cards. They have different kinds of power. Some cards are versatile and possess multiple kinds of power, like Rishadan Port.
Searing Wind packs more power than Lightning Bolt, but is worlds less efficient (not always true though, Opportunity looks better than Ancestral Recall under Dream Halls). I'd also say that Yawgmoth's Bargain is more powerful than Necropotence. It's also more expensive.
Being that speedy mana acceleration allows one to utilize all this power in cards, many people rank Black Lotus as the most powerful of cards. I'd say that it's a good candidate for the position.
I hesitate to put Necropotence (or any card) on a pedestal as most powerful, because of the variation in the game. I will say that it is probably the best card drawing engine ever.
