For example, blue provides card draw with cards like Consecrated Sphinx and blue countermagic like Mana Drain or Counterspell, black offers powerful spot removal like Fatal Push and has access to the best tutors in the game in the form of Demonic Tutor or Vampiric Tutor, and white adds board wipes (Wrath of God/Day of Judgment) and recursion in cards like Second Sunrise or Faith's Reward
Rise of the Witch-king
By now most serious buyers know the right places to park their resourcesPower, Duals, highly graded Alpha and Beta cards, Reserved List, etc
It doubles your +1/+1 counters for huge swings while stifling your opponents on the same front
7 Heart Of Kiran Heart of Kiran is another appealing Kaladesh plane, first introduced in the Aether Revolt set
You have your typical Elvish Mystics and Fyndhorn Elves, but also your high-netting mana dorks like Canopy Tactician and Llanowar Tribe, which can leave you with 7-8 mana on turn 3 or 4