We need to put a "WHY" by cards to de-poll-ize this thread.
Lucario GL = 1 in each of your decks will allow you to DOUBLE your opponents weakness if possible. This could lead to a game-saving 1 shot knock out of your opponents giant!
Super Scoop Up = it's a flip, but it gets the active up and out of the way if you don't have a warp point or switch. THe most OBVIOUS reason you'd use this card is to pull up a damaged pokemon and, therefore, waste your opponents attacks.
Dusk Ball = adds speed to the deck. It gives you 7 cards to choose from. If you can't find something in there you need, your build may be to varied.
If we are just talking about staple cards that can be used in most decks . . .
Absolute Must Haves
4x Roseanne's Reasearch
4x Bebe's Search
2x Night Maintenance
1x Luxury Ball
Very Nice to Have
2x Warp Point
4x Rare Candy
4x Broken Time-Space
2x Cynthia's Feelings
4x A good draw Supporter (Volkner's Philosophy, Felicities Drawing, or Professor Oak's Visit)
Needed for Tournament Decks
2x Baltoy GE
2x Claydol GE
2x Uxie LA
1x Azelf LA
2x Unown G
After that it gets more specific according to what kind of deck you want to make.