3 mana for a 3 loyalty planeswalker is also an excellent rate, allowing you to play Aminatou early for access to an extra card in a pinch, providing easy setup of an inexpensive permanent with a relevant ETB ability and Aminatou on the same turn, or simply letting you play Aminatou late with mana up to protect it from countermagic or removal
Given how game-winningly nasty that ultimate, might I suggest Green for the counter adding and manipulation of Doubling Season et all
Blue likes to wreak and play havoc with your strategies and game plan
Lifegain from Leonin Vanguard and Vampire tokens quickly goes from incidental to real numbers
Honorable Mentions: Magus of the Moon
Getting multiple activations to kill again each turn is crucial, so I recommend Thousand-Year Elixir, Illusionist's Bracers, or Battlemage's Bracers