For an in-depth look at the best path for collecting all the goodies across the set, point interested staff (and players in your community) to the Collecting Modern Horizons 3 article on DailyMTG
Your starting deck in Commander cant exceed 100 cards, so Yorion, Sky Nomad is as your companion because you can never have a legal 120-card deck in the format
Zodiac Dragon ( F, MC, C ) - This is an 8 mana 8/8, which returns it to your hand if it dies
Morgul-Knife (Shadowspear) Exxaxl: Shadowspear was one of the cards I first noticed becoming expensive because of Commander
This one is especially brutal, I have lost a lot of games to people using spot removal on their own stuff to wreck a Treachery turn
Bringing them back as non-creature enchantments lets you leverage lots of powerful abilities