Which cards are most bought by players, and most often associated with winning decks?
It's currently down but it should give you the most accurate answer when it goes back up again.
For example, here is a ranking of the best openings from google's cache
The card that immediately pops to mind:
Here's an article on dominionstrategy.com which outlines why this card is considered the best in the game. No other card allows you to shape your deck so quickly and early, ensuring you get useful and powerful draws to ramp up your deck.
If you look at the best openings ranking linked by Dor Shemer, you'll see many openings at the top of the list that include Chapel.