Beast Within is the best green removal spell
Brawl Legal Vintage Legal Timeless Legal Commander Legal Pauper Not Legal Oathbreaker Legal Penny Legal Memorial to War Land This land enters tapped
Herald's Horn and Heirloom Blade may seem innocuous, but every future tribal deck can play them
But if the Bubble Smuggler gets face-down for some other reason, like if it was Cloaked or Manifested, then you can turn it up for 2 mana (as Manifest Dread says) and it will still notice it got turned face up, getting those counters
Technically, Elvish Mystic is also only worth around $0.34, yet the Hatsune Miku variant sells for a lot more
All the other Arena-exclusive cards could very well have cardboard counterparts if Wizards wanted, which makes them pretty inconsequential to the discussion