When building a Deck, do not just settle for 1 of a really good card. As long as the card is not Limited or Semi-Limited, why not put 3 in your Deck?
If you have a favorite card and it works well for your when you Duel, see if you can get 2 or 3 copies of it and play with all of them. Remember you can not have more than 3 copies of a card in your Deck.
You will not always be able to fit 3 of every card into your Deck. You will need to make choices on which cards work better for you than others. If you put in 3 copies of every card you want, your Deck might get too big. Sometimes you will have to settle for 1 or 2 copies of a card you really like.