The sets chase is primarily Booster Fun treatments (Japan Showcase and its premium foils) rather than one single must-open card
Of course, it also has pack tactics, which reads, Whenever Minion of the Mighty attacks, if you attacked with creatures with total power 6 or greater this combat, you may put a Dragon creature card from your hand onto the battlefield tapped and attacking. Nobody likes a surprise Dragon except the player taking advantage of it
The other option brings us to number one on my list
You cant set one preferred printing for Explorer and one for Standard, for example
Gardevoir ex Gardevoir ex's Psychic Embrace ability is just so powerful at moving energy around that it's made this deck a force on the competitive scene for years
An interesting side effect of hybrid mana is that it lets you treat heavy mana costs in two-color decks like you normally treat monocolor