This is stated in the spells text
Research recent sales data and current market values, assess the condition of your cards, and consider their rarity and demand
Good trap cards are those that can: prevent attacks like Mirror Force, negate summons like Solemn Warning, negate spell/trap activations like Wiretap, negate effects like Breakthrough Skill or Fiendish Chain, remove monsters like Ring of Destruction or Compulsory Evacuation Device, and prevent your opponent from making plays like Vanity's Emptiness
When Edgar Markov decides to join the battle, he comes out the gates swinging, immediately attacking for 5 and buffing your Vampire army in the process -- the perfect follow-up to all the free tokens he's been adding to your army
Its also probably the easiest to upgrade as both ink colors have good reasonably-priced options or very expensive but excellent ones