Doubling Season is one of the few cards of its type that can double the counters that are placed on planeswalkers, letting you use their final and often strongest abilities as soon as they come into play
Drawing all three and keeping them on the board takes insane luck
A 5/5 Human Knight with vigilance and menace for six mana, whenever an equipped creature under your control attacks, you can draw a card for the low cost of losing one life, potentially allowing several additional cards to be drawn in a single turn
Its versatility, especially with how well it lines up Unholy Annex, makes it such an auto-include into any Green strategy, regardless of if you're stompy or midrange
Besides its own impressive statline and relevant combat abilities, I love that its assassin-creation ability doesnt specify one or more, and instead chunks out a 1/1 Assassin creature for each historic creature that dealt damage