It allows for easy organization and viewing of the collection
But if you can only buy a single one right now, the answer is the Innistrad Remastered collector booster
Shantotto, Tactician Magician A limited focused card for the UR cast big spells archetype which seems like a perfectly decent little dude, who also loves affinity for artifacts
In addition, the census of any particular card can and does change as new cards are graded and/or discovered
When talking about higher power levels, Id be remiss to omit Mana Crypt
For example, FF4 's Cecil and Kain both have their stories told in their respective cards - Cecil starts as a dark knight that loses the owner life and transforms into a paladin with lifelink to restore what was lost, whereas the complex character that is Kain betrays the party and becomes an enemy, which is represented by being a card that changes owners