A Must, Scyther
With the Jungle Expansion came its most important card, Scyther. Scyther's versatility is almost unmatched and provided an improvement for the previous all purpose card, Farfetch'd. To quickly summarize Scyther, he is a Leaf type Pokemon with 70 Hit Points, Resistance to Fighting, NO retreat cost, and weakness to Fire. He has two attacks, Swords Dance for 1 Leaf Energy doubles the damage of his Slash attack, a 30 damage attack requiring 3 Colorless Energies.
His most important characteristic seems to be his resistance to Fighting. A few cards that are resistant to Fighting should be in every deck, and Scyther fits that role very well. A fully powered Hitmonchan will do only 10 damage to Scyther, making him nearly meaningless. Scyther's weakness to Fire is fairly good, due to the low popularity of Fire decks.
The second most important, if not the most important, characteristic is the Free Retreat cost. This characteristic is extremely important and allows great flexibility. Read my article on Free Retreat for a greater view and potential of this ability.
His Leaf Pokemon type is also something that should not be overlooked. His type allows non-Leaf carrying decks to have a card that can attack as Leaf, although not requiring Leaf Energy. This is very important for Lightning decks, as all Lightning resistant Pokemon are weak to Leaf Pokemon. Also, many of the Water Pokemon are weak to Leaf as well, like Gyrados and Poliwrath. Scyther also provides an answer to Aerodactyl, the anti-evolving monster.
His Slash attack is fairly useful. It's not the most effective, but it only requires Colorless Energy. So you can play a basic energy card and a Double Colorless for an instant attack. This attack allows him to be useful in any deck.
Swords Dance is an extremely useful attack if you have Leaf Energy. Most newer players generally don't know how to use this. There are a few ways, first, when you have two one card on him, use Swords Dance. The next turn, add another energy and attack. Or, when you have one energy, Swords Dance and play a Double Colorless the next turn for a turn two 60 damage attack.
But that's not where the strategic importance of the attack is. Coupled with Gust Of Wind, Swords Dance is an extremely useful attack. Let's say the opponent has one Blastoise left on the bench with 50 damage. You play Scyther and Swords Dance, even though you have enough energy to Slash. The next turn, use Gust Of Wind on Blastoise and Slash for 60 damage. If you used Gust Of Wind first and Slash once for 30, the opponent can still retreat Blastoise or use a Switch Trainer to get the old man out. Since very few Pokemon can KO Scyther in one attack, Swords Dance proves fairly valuable from time to time.
Swords Dance is also very useful against energy draining Pokemon such as Butterfree or Rage attacking Pokemon like Dodrio and Tauros. Swords Dance is useful against defensive Pokemon like Chansey. Whenever Chansey successfully scrunches, just Swords Dance.
Scyther may not be valuable in ALL decks, but he is quite versatile and useful in most decks. Next time you're building a deck, consider the workhorse himself, Scyther.