A great strategy in dominion is to get rid of your bad cards so your better ones come up more often. At the beginning of the game you start off with a only a few weak cards, so if you can trash some of them and buy a stronger card it will come up very often and you'll have less weak cards later in the game.
Some of the strongest opening cards I've used:
- Chapel. Trash as many copper and estates as possible, even if you don't get to buy much. If chapel is in play my first 2 buys are almost always a chapel and a silver.
- Money Lender. Trash a copper from your hand to get +3$. Ideal for openings.
- Coppersmith. Make all that copper crank out double!
- Trading Post. Trash two cards and get a silver for free.
- Treasure Map. If you have 2 in your hand, trash them to get 4 gold on top of your deck. Early game if the easiest time to do this usually. throw in a +cards card to get the most bang for your buck.
There are tons of cards that are good in openings, like most attacks and most cards that let you draw more than 2 cards, but I've found these are some of the absolute strongest that tend to dominate openings whenever they're in the cards being used.