Dialga G LV X has impacted the competitive game in a big way. Though it's lost some steam since cities, Dialga G remains a popular tech for SP decks, a good tech for some non-SP decks, and it's even used on it's own.
I suspect that Dialga G LV X is a victim of it's own popularity. This card was huge at cities, and it's use led to the decline of pokemon reliant on PokeBodies, and some decks (like Scizor) practically vanished. Since then, many decks considered the possibility of encountering a Dialga G, and the use of pokemon reliant on PokeBodies declined. Thus, Dialga G likewise declined. It was like an intellectual checkmate between PokeBodies and Dialga G, it's mere existence was enough to merit second thought. However, the use of Spiritomb from Arceus is good reason to at least run this card as a tech.
When attacking with this, use Special Metal Energies if you can. Energy Gain will give you a turn one deafen which does 10 and helps to stall your opponent, who probably wanted to play Trainer or Stadium cards. Turn two, play Dialga G LV X, give it another Special Metal Energy, play Crobat G from your hand so the opponent's pokemon has 2 damage counters so you can do 70 more with Second Strike. From the third turn onward, use Remove Lost to do 80 every turn, and the opponent risks losing their precious energy cards... permanently.
One popular pokemon to use with this card is... Actually, it can fit well in nearly any SP deck, but my own favorite is Garchomp C. Dialga G is often used as a main attacker. When this is the case, it tends to pile up damage counters. Thankfully, it gets the benefit of Special Metal Energy, and it can take less damage from attacks. When Dialga G LV X is close to biting the dust, retreat or use a Switch (or Warp Point), and level up Garchomp C. Healing Breath takes care of all the damage. If you were to use a PokeTurn, you'd have to play all that energy again, but with Garchomp C, Dialga G remains a constant threat.
Alright, now let's kill this card. Dialga G does not like Blaziken FB. Blaziken's attack makes it active and burns it, which Dialga G probably won't like. With a retreat cost of 2, Dialga G isn't likely getting away, and if it is, the opponent is discarding 2 energy to do so. And on the next turn, Blaziken FB levels up and kills it. If the opponent has a water type (like Palkia G) in play, Vapor Kick will do it. If not, you'll need Jet Shoot if Dialga G hasn't taken enough burn damage. Charizard from Arceus would have to use Burning Tail. Dialga G would shut off it's Fire Formation PokeBody, so Fire Wing would not be enough in most cases. For killing Dialga G outright, Ninetales from Mysterious Treasures is my favorite card, which also takes care of Mewtwo LV X (so does Dialga G, incidentally).
Dialga G LV X, for providing a powerful tool for cheesy PokeBodies, gets a score of 4 out of 5.
If the most popular decks in the Modified format were more dependent on PokeBodies, I would have rated Dialga G LV X higher. But then, Dialga G LV X is the main reason why most of those decks aren't being played. So in that sense, Dialga G LV X will continue impacting the game until the day it is rotated out.
